Fix a11y for tappable BiometricPrompt empty space

Test: manual:
1. Enable TalkBack
2. Open the BiometricPrompt demo app
3. Press "Authenticate" from any demo activity

Before: TalkBack reads "Tap to cancel authentication" twice
After: TalkBack reads "Tap to cancel authentication" once

Fixes: 149545007
Change-Id: I2ae6aa65e7e2dec2dca6325074556e4db10c7880
1 file changed