Fix NullPointerException in aspect ratio restrictions

In this CL we properly clear inherited letterbox behavior of
translucent activities when the ActivityRecord source or the
inherited activity is removed.
If this doesn't happen, onMergedOverrideConfigurationChange()
is triggered on an ActivityRecord with no TaskFragment associated
then the null pointer exception.

Fixes: 272941700
Test: Reproduce steps on bug description
      Run `atest WmTests:SizeCompatTests`

Change-Id: Ifeeb3042c6672e32d8cd665e9cc2f9abaf558a9c