Improve workaround for partial texture updates on Android

Improve workaround for partial texture updates on Android

* java/AndroidManifest.xml.in:
* java/org/gnu/emacs/EmacsDialog.java (toAlertDialog): Don't
change hardware acceleration state.
* java/org/gnu/emacs/EmacsNative.java (notifyPixelsChanged): New
function.
* java/org/gnu/emacs/EmacsSurfaceView.java (EmacsSurfaceView):
New field `bitmapChanged'.
(copyToFrontBuffer): Signal that the bitmap has changed.
(onDraw): If the bitmap has changed, increment the generation
ID.
* src/android.c (JNICALL): Implement new function.