[flexiglass] Addresses comments from ag/23884810

1. Defaults AuthenticationRepositoryImpl.isAutoConfirmEnabled to false
   instead of true.
2. Moved hintedPinLength test cases from PinBouncerViewModelTest to
   AuthenticationInteractorTest.

Bug: 280883900
Test: All unit tests still pass
Test: Manually ran on device, made sure that the PIN bouncer works with
autoconfirm on and off.

Change-Id: I6c7b876a82ead8698723b0e53e69c7edff198fbc