Fix Continous loop in unifed screenlock when trying to Trust a CA cert

Cause: with unified screenlock, ConfirmDeviceCredentialActivity didn't
forward result with FLAG_ACTIVITY_FORWARD_RESULT

Also, fixed that ConfirmDeviceCredentialActivity didn't allow fingerprint
authenication in unified screenlock after keystore unlocked.

In ChooseLockSettingsHelper, add one new util function to allow
extra option to set returnCredentials to false while external to true.

Set StrongAuth to "not required" when it has been successfully unlocked.

Test:
1. PO Unified Screenlock/Work Challenge x fingerprint -> ok to trust cert
   (Also, no credential is returned in intent)
2. WorkMode off -> Reboot -> turn on Work mode
   -> no fingerprint option, PIN unlock successful to turn work mode on

Bug: 28752364
Change-Id: I6dc8865e8f005545f8577d7731afb4495647062b