Keep track of whether LockscreenCredential is a unified profile password

Unified profile password LockscreenCredential objects are created only
using LockscreenCredential#createUnifiedProfilePassword(), but until now
they didn't carry any state to differentiate them from regular
passwords.  But several places do need to differentiate them from
regular passwords.  Workarounds are being used currently: passing around
a boolean separately, or assuming that very long passwords are unified
profile passwords.  Let's solve this by adding a
mIsUnifiedProfilePassword field to LockscreenCredential.

(We should still make it a different credential type sometime, but that
would be a larger change because the credential type is stored on-disk.)

Test: atest FrameworksCoreTests:LockscreenCredentialTest
Bug: 417229857
Flag: EXEMPT not possible
Change-Id: I9d349284cb826b0b5bd59086578e4e50c56ef490