Stopgap fix for credential change of unified-challenge profile

LockSettingsService regressed in Android 11 that it no longer always
discard the original credential parameter when performing credential
changes for unified-challenge profile. Add a stopgap fix to restore
that behaviour and document idea for long term fix.

Bug: 147663267
Test: Fire ACTION_SET_NEW_PASSWORD from profile to set a separate
      work challenge. Verify the flow succeeds.
Change-Id: Idc7f67c6cd021a779fdcd1992cd5cbbd337c4e52