WallpaperPicker: Destroy cropview to avoid leaks

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: I0804c903d14d3f3d13529f0fe90d12784d97e93f
(cherry picked from commit c91c765c10ad7ccab0092ad369e2941bf7ef2e93)