Wait 2 frames before hiding shared elements.

Bug 15991516

Because of double and triple buffering, we can't guarantee
that the called Activity will show shared elements prior to
the calling activity hiding them. This forces the called
Activity to wait 2 frames prior to telling the calling activity
to hide its shared elements.

Change-Id: Ia49e71637ea04f8d7f93093abf6f47b00dcc7c06