Fix BiometricPrompt double TalkBack announcement

Fixes an issue that caused the content description for the
BiometricPrompt background cancellation target to be read aloud twice by
TalkBack when the prompt was displayed.

Test: Manual:
1. Enable Talkback
2. Install the CtsVerifier apk
3. Launch app and select Biometric Tests > 4a: Cipher, Credential
4. Select "time-based key with biometric" to launch prompt

Before: TalkBack reads "Tap to cancel authentication. Tap to cancel
authentication, button."

After: TalkBack reads "SystemUI. Tap to cancel authentication, button."

Fixes: 155042588
Change-Id: I93f7380d87213aba8f1e1030510b84170ae9c2f7