Update Biometric Prompt Credential screen styling

Adds a back button for the credential only case and updates the edit text styling

Test: Verified manually
Test: atest CredentialPatternViewScreenshotTest
CredentialPasswordViewScreenshotTest
Bug: 437413510
Flag: EXEMPT bugfix

Change-Id: Ia7b175c2129abff9f7823a2d6836f830f24c4e82