Check for null receiver in biometric AuthController

We've witnessed at least one crash due to mReceiver being null while
trying to handle callbacks such as onTryAgainPressed in AuthController
(formerly BiometricDialogImpl). In these cases, not running the callback
is likely better than crashing, so this commit adds null-checks where
they're currently missing. It also adds unit test cases to exercise the
new logic.

Test: atest AuthControllerTest
Fixes: 143251337
Change-Id: I3c46c7fab0dfe802de499180c7bb0c756f7e652b