Trebuchet : Destroy cropview to avoid leaks in picker

If the cropview's TiledImageRenderer is rendering threads, the activity
context gets leaked. Avoid that by invoking its destroy in onDestroy() of activity.

Change-Id: I8503e88d1cab7fc8dab61ba7da148579902f0477
(cherry picked from commit 9319afbea6b17e42bcee0716a7d08caccec880df)