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

core/java/android/window/WindowOnBackInvokedDispatcher.java

@@ -291,6 +291,7 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
                 mProgressAnimator.reset();
                 final OnBackInvokedCallback callback = mCallback.get();
                 if (callback == null) {
+                    Log.d(TAG, "Trying to call onBackInvoked() on a null callback reference.");
                     return;
                 }
                 callback.onBackInvoked();