Fix IME flicker to home screen w/ orientation change

In [1] we attempted to updateImeParent() onConfigurationChange(),
computeImeParent() can pick a parent that didnt even request IME.
This can cause flicker when going from app to launcher.

Adding a check for prevents this from happening.

Fix: 188864578
Test: atest DisplayContentTests

Change-Id: I01b67b9a7fdf04f23ab05c244281f42518352ca5