Prevent Android OS task trimming from deleting Emacs frames

Prevent Android OS task trimming from deleting Emacs frames

* doc/emacs/android.texi (Android Windowing): Document proviso
on Android 7.0 and later.

* java/org/gnu/emacs/EmacsActivity.java (EmacsActivity)
<timeOfLastInteraction>: New field.
(onStop, onResume): Set and clear timeOfLastInteraction.
(isReallyFinishing): New function.
(onDestroy): Don't delete frame even in the event isFinishing
returns true if more than 4 hours have elapsed since the
activity last moved into the background.

(cherry picked from commit 755665d95adbba07335f400f1090e53b66c41ff5)