Update sign-in titles

1. Remove question marks to be consistent with the single tap biometrics
   titles that also don't have question marks
2. Replace 'sign-in' with 'account' for mixed / unknown credential
   presentations.

Test: see screenshot in bug
Bug: 334170176
Bug: 334108777
Change-Id: Id3598bd0d244d3e7608a8b27f0635175118eb4c8