[Biometrics] Ensure AuthSession propagates fallback option dismissals to client.

AuthSession#onDialogDismissed previously lacked specific handling for DISMISSED_REASON_FALLBACK_OPTION_BASE. This prevented the client's OnClickListener (set via BiometricPrompt.Builder#addFallbackOption) from being invoked. This change adds the required case, ensuring AuthSession calls mClientReceiver.onDialogDismissed(reason). This enables the client-side BiometricPrompt to execute the correct fallback listener.

Bug: 419100262
Flag: EXEMPT bug fix
Test: atest AuthSessionTest
Change-Id: I48d7e8994c08df5439f3bb2c1a90d3d7121b7d6d