15/n: Allow Auth UI to start in credential UI

If the user is locked out of biometrics, and
BiometricPrompt#setDeviceCredentialAllowed(true), the user should be
shown the credential UI.

This change gives BiometricService the ability to request SystemUI
to show AuthCredentialView without first showing AuthBiometricView.

Bug: 140127687

Test: atest BiometricServiceTest
Test: atest com.android.systemui.biometrics

Change-Id: Ic26986ba044b7992641676c3d3b99fc1395a45b7