Enable customization of the quit key on Android

Enable customization of the quit key on Android

* doc/emacs/android.texi (Android Windowing):

* doc/emacs/input.texi (On-Screen Keyboards): Document various
tidbits related to the quit key.

* java/org/gnu/emacs/EmacsNative.java (getQuitKeycode): New
function.

* java/org/gnu/emacs/EmacsWindow.java (EmacsWindow): Rename
`lastVolumeButtonRelease' to `lastQuitKeyRelease'.
(onKeyUp): Treat value returned by getQuitKeycode as the quit
key rather than mandate KEYCODE_VOLUME_DOWN.

* src/android.c (getQuitKeycode): Implement new function.

* src/androidterm.c (syms_of_androidterm)
<android_quit_keycode>: New variable.