Fix reporting of key events containing SYM and META

Fix reporting of key events containing SYM and META

* doc/emacs/android.texi (Android)::(What is Android?):
(Android Startup, Android File System, Android Environment)
(Android Windowing, Android Fonts, Android Troubleshooting):
Improve section titles.
(Android Windowing): Describe the relation between keyboard
modifiers reported by Android and those in key events.
* java/org/gnu/emacs/EmacsWindow.java (onKeyDown, onKeyUp):
Clear META_SYM_ON and META_META_MASK when retrieving ASCII
characters.
* src/androidgui.h: Add ANDROID_META_MASK.
* src/androidterm.c (android_android_to_emacs_modifiers)
(android_emacs_to_android_modifiers): Transform META to Alt, and
vice versa.