Improve a11y for BiometricPrompt credential view

Ensures text that is important for accessibility in the auth prompt
credential view is read aloud by TalkBack.

Test: Manual:
1. Set PIN/pattern/password as screen unlock method
3. Enable Talkback in settings
4. Launch the CtsVerifier-v2 test app
5. Open SECURITY > Biometric Tests > BIOMETRIC+CREDENTIAL TESTS
6. Tap 4a: SetUserAuthParams: Credential, Cipher
7. Tap Choose Unlock time-based key with credential
8. Enter an incorrect PIN/pattern/password

Fixes: 151389440
Fixes: 151390960
Change-Id: I95c38cb678b0ae46988539b167c48fe9572677bc