Clear UDFPS BiometricPrompt a11y messages after read by talkback

Clears UDFPS guidance messages after focus moves to the next view, in
order to prevent stale messages during any subsequent linear a11y
navigation, and to allow sending the same message multiple times in
succession (e.g. "Move left" twice in a row)

Flag: NONE bug fix
Fixes: 404940015
Fixes: 404938119
Fixes: 383230658
Test: (manual) verified Talkback reads expected messages
Test: atest PromptViewModelTest
Change-Id: I685ae0abe2c8df017c06187252eb8e26d56d3113