Add internal API for credential text in BiometricPrompt

Adds a hidden API behind an internal permission check that allows the
Settings app to use specific text to BiometricPrompt for credential
auth. This allows the work authentication prompt to use work-specific
language when verifying work credentials, while retaining more generic
language when authenticating with biometrics.

Test: Work lock prompt shows "Enter your work ___" for credential
Test: Work lock prompt now shows "Verify it's you" for face

Bug: 149003660
Change-Id: Idf06f7e008f4bdb448b0d539522efd2e9290a3bb