Fix exception from DateTimeView unregisters receiver again

Although parent view may invoke onDetachedFromWindow incorrectly,
itself should guarantee to unregister receiver only when removing
the last attached view.

Change-Id: I4b701c223c6aa354896a4067f77aad9c98d88dfd
Fixes: 78506836
Test: atest FrameworksCoreTests:DateTimeViewTest
2 files changed