Register fallback even if background already exists

Fallback would not be registered/drawn if background was explicitly set before calling
setContentView and the area that was supposed to be covered by fallback ended up black.
mBackgroundDrawable does not control fallback, so setting fallback should not be dependent on
it's existence.

Change-Id: I7da85e8c61dababa3a7ce930238ea0f283d19b32
Fixes: 68822553
Test: atest PhoneWindowTest