Docs: Add Android platform documentation

How it works page, host environment variables page,
and relevant CMake information page added.

Fixes: QTBUG-107704
Pick-to: 6.4 6.2
Change-Id: I823d3f9d97f49e79da867660bd4cd41a8378266a
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

doc/config/qtdoc.qdocconf

@@ -65,7 +65,8 @@ qhp.QtDoc.subprojects.examples.sortPages = true
 # Add an image used in example manifests to qhp, as well as thumbnails for YouTube links
 {HTML.extraimages,DocBook.extraimages,qhp.QtDoc.extraFiles} += images/qt-codesample.png \
                                                                images/yIv0vO8B7tQ.jpg \
-                                                               images/5OiIqFTjUZI.jpg
+                                                               images/5OiIqFTjUZI.jpg \
+                                                               images/nmvurCcsWos.jpg
 
 # Add an .html file with sidebar content, used in the online style
 HTML.stylesheets             += style/qt5-sidebar.html

doc/src/platforms/android/android-CMake-variables.qdoc

@@ -0,0 +1,34 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
+
+/*!
+\page android-build-environment-variables.html
+\title Qt for Android: CMake
+\brief Information regarding CMake and Android platform APIs.
+\ingroup androidplatform
+
+This page lists information regarding CMake and Qt for Android platform support.
+
+\section1 Variables affecting the build process
+
+    \list
+        \li \l{QT_ANDROID_PACKAGE_SOURCE_DIR}
+        \li \l{QT_ANDROID_EXTRA_LIBS}
+        \li \l{QT_ANDROID_EXTRA_PLUGINS}
+    \endlist
+
+\section2 Variables affecting Gradle
+    \list
+        \li \l{QT_ANDROID_MIN_SDK_VERSION}
+        \li \l{QT_ANDROID_TARGET_SDK_VERSION}
+        \li \l{QT_ANDROID_SDK_BUILD_TOOLS_REVISION}
+    \endlist
+
+\section2 Variables affecting the Android app manifest file
+    \list
+        \li \l{QT_ANDROID_VERSION_CODE}
+        \li \l{QT_ANDROID_VERSION_NAME}
+        \li \l{QT_ANDROID_APPLICATION_ARGUMENTS}
+    \endlist
+
+*/

doc/src/platforms/android/android-envoirnment-variables.qdoc

@@ -0,0 +1,44 @@
+// Copyright (C) 2020 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
+
+/*!
+\page android-environment-variables.html
+\title Environment Variables Used by Qt for Android
+\brief Lists some useful environment variables used by the Qt For Android toolchain.
+\ingroup androidplatform
+
+\section1 Enabling or disabling workarounds
+
+Qt Android uses some environment variables to enable/disable certain workarounds:
+\section2 Commonly used variables
+\list
+    \li QT_ANDROID_NO_EXIT_CALL: In some cases, an Android app might not be able
+    to safely clean all threads while calling \c exit() and crash. This is because
+    there are C++ threads running and destroying these without joining them
+    terminates an application. These threads cannot be joined because it's not
+    possible to know if they are running.
+    This flag avoids calling \c exit() and lets the Android system handle this,
+    at the cost of not attempting to run global destructors.
+    \li QT_ANDROID_ENABLE_WORKAROUND_TO_DISABLE_PREDICTIVE_TEXT: Allows
+    the hiding of predictive text suggestions, mainly from password fields. Certain
+    devices don’t handle this properly.
+    \li QT_ANDROID_ENABLE_RIGHT_MOUSE_FROM_LONG_PRESS: Interprets a long touch press
+    as a right mouse click event.
+    \li QT_ANDROID_DISABLE_ACCESSIBILITY: Disable \l{Assistive Tools}{Accessibility}.
+    This prevents the processing of accessibility events.
+\endlist
+
+\section2 Less commonly used variables
+\list
+    \li QT_ANDROID_FONT_LOCATION: Sets a custom path for system fonts.
+    \li QT_ANDROID_MAX_ASSETS_CACHE_SIZE: cache size for assets under the
+    assets folder.
+    \li QT_ANDROID_DISABLE_GLYPH_CACHE_WORKAROUND
+    \li QT_ANDROID_RASTER_IMAGE_DEPTH
+    \li QT_ANDROID_MINIMUM_MOUSE_DOUBLE_CLICK_DISTANCE
+\endlist
+
+\note Though related, these are not the same as CMake commands or variables used
+by Qt For Android, see \l{Qt for Android: CMake}
+
+*/

doc/src/platforms/android/android-how-it-works.qdoc

@@ -0,0 +1,56 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
+
+/*!
+\page android-how-it-works.html
+\title Qt for Android: How It Works
+\brief An overview of architecture, toolchain, and other useful information.
+\ingroup androidplatform
+
+
+If you're a developer looking for a high level overview of how Qt supports the
+Android platform, this page is for you.
+
+The video from the 2021 Qt World Summit gives an overview of Qt for Android.
+
+\youtube nmvurCcsWos "A picture of an agenda that links to a YouTube video"
+
+\section1 Architecture
+
+\section2 Activities
+The implementation of Qt for Android is made up of three main parts,
+and the glue in between:
+
+\list
+    \li Qt Core
+    \li the Android QPA plugin
+    \li The Java Android code
+\endlist
+
+\image qt_android_architecture.png "An overview Qt for Android's Architecture"
+
+\list
+    \li The default entry point for a Qt app is the QtActivity, which upon
+    starting, will call the QtActivityLoader which will try to load the
+    application by calling the QtActivityDelegate.
+    \li QtActivityDelegate will make the necessary calls to load the Qt
+    libraries and the main app library with the help of QtNative.
+    \li QtNative handles the various Java operations that don’t need to be
+    exposed, such as: loading the libraries and handling files.
+    \li The QtActivityDelegate also handles the different listeners and input
+    handlers and propagate them from Java to Qt.
+\endlist
+
+\section2 Services
+
+The architecture here is similar to that of Activities, but with services
+specific classes.
+
+For information on how Qt supports Services in Android see \l{Android Services}.
+
+\section1 Build configuration
+
+For more information on configuration Qt for Android Builds see
+\l{Environment Variables Used by Qt for Android} and \{Qt for Android: CMake}.
+
+*/