Document all the environment variables used by Qt for Android

Qt for Android uses or excepts some environment variables to enable some workaround in some scenarios, but those are not documented. Those env vars should be documented and their purpose and behavior described for users. 

List of variables, grouped by a common file in wich they were used or set

QtActivityDelegate.java

QT_ANDROID_ENABLE_WORKAROUND_TO_DISABLE_PREDICTIVE_TEXT

QT_ANDROID_APP_ICON_SIZE

QT_ANDROID_FONTS_MONOSPACE

QT_ANDROID_FONTS_SERIF

QT_ANDROID_FONTS

QT_ANDROID_VOLUME_KEYS

QtActivity.java

QT_ANDROID_THEMES

QT_ANDROID_DEFAULT_THEME

QT_ANDROID_THEME_DISPLAY_DPI

QtLoader.java

QT_ANDROID_THEMES_ROOT_PATH

qopenglcontext.cpp

QT_ANDROID_DISABLE_GLYPH_CACHE_WORKAROUND

androidjniinput.cpp

QT_ANDROID_ENABLE_RIGHT_MOUSE_FROM_LONG_PRESS

androidjnimain.cpp

QT_ANDROID_NO_EXIT_CALL

qandroidassetsfileenginehandler.cpp

QT_ANDROID_MAX_ASSETS_CACHE_SIZE

qandroidplatformscreen.cpp

QT_ANDROID_RASTER_IMAGE_DEPTH

qandroidplatformtheme.cpp

QT_ANDROID_MINIMUM_MOUSE_DOUBLE_CLICK_DISTANCE