Some cleanup logging and test for back navigation

 - Better logging
 - Remove method form OnBackInvokedDispatcher interface (but not the
   implementation)
 - Add test for ag/17211289

Bug: 222675481
Test: BackNavigationControllerTests#testUnregisterCallbacksWithSystemCallback
Change-Id: I6a3ae30da7e29dddda116acd8fa0c9af4b9faa6e