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.

java/org/gnu/emacs/EmacsNative.java

@@ -228,6 +228,10 @@ public final class EmacsNative
      be prevented from reaching the system input method.  */
   public static native boolean shouldForwardCtrlSpace ();
 
+  /* Return the keycode repeated activation of which should signal
+     quit.  */
+  public static native int getQuitKeycode ();
+
   /* Initialize the current thread, by blocking signals that do not
      interest it.  */
   public static native void setupSystemThread ();