DO NOT MERGE Add logging for when IOnBackInvokedCallback is null.

It should never be null. We had a bug where the IME callback can be
unexpectedly GC-ed and jam back nav. This is to verify if we need its
fix ag/21301891 in QPR as well.

Bug: 274911901
Test: atest WindowOnBackInvokedDispatcherTest
Change-Id: Ic2a576655ca16577ca35f3544f5b26d4a0db8f90