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.

2 files changed, 23 insertions, 0 deletions

diff --git a/src/android.c b/src/android.c
index e44b58c5973..00a77fc398d 100644
--- a/src/android.c
+++ b/src/android.c
@@ -2645,6 +2645,13 @@ NATIVE_NAME (shouldForwardMultimediaButtons) (JNIEnv *env,
   return !android_pass_multimedia_buttons_to_system;
 }
 
+JNIEXPORT jint JNICALL
+NATIVE_NAME (getQuitKeycode) (JNIEnv *env, jobject object)
+{
+  /* Likewise.  */
+  return (jint) android_quit_keycode;
+}
+
 JNIEXPORT jboolean JNICALL
 NATIVE_NAME (shouldForwardCtrlSpace) (JNIEnv *env, jobject object)
 {
diff --git a/src/androidterm.c b/src/androidterm.c
index e4f3abdb2d3..5de7b6f4e14 100644
--- a/src/androidterm.c
+++ b/src/androidterm.c
@@ -6703,6 +6703,22 @@ so it is important to limit the wait.
 If set to a non-float value, there will be no wait at all.  */);
   Vandroid_wait_for_event_timeout = make_float (0.1);
 
+  DEFVAR_INT ("android-quit-keycode", android_quit_keycode,
+    doc: /* Keycode that signals quit when typed twice in rapid succession.
+
+This is the key code of a key whose repeated activation should prompt
+Emacs to quit, enabling quitting on systems where a keyboard capable of
+typing C-g is unavailable, when set to a key that does exist on the
+device.  Its value must be a keycode defined by the operating system,
+and defaults to 25 (KEYCODE_VOLUME_DOWN), though one of the following
+values might be desired on those devices where this default is also
+unavailable, or if another key must otherwise serve this function
+instead:
+
+  - 4  (KEYCODE_BACK)
+  - 24 (KEYCODE_VOLUME_UP)  */);
+  android_quit_keycode = 25;
+
   DEFVAR_BOOL ("x-use-underline-position-properties",
 	       x_use_underline_position_properties,
      doc: /* SKIP: real doc in xterm.c.  */);