Add RemoteLockscreenValidationFragment to help retain remote lockscreen

validation state.

Currently, if ConfirmDeviceCredentialBaseFragment is ever re-created due
to orientation change, screen getting turned off, etc., relevant state
gets lost. This led to the old ConfirmDeviceCredentialBaseFragment
handling results which led to issues such as lockscreen not getting set.
By addiing a retained RemoteLockscreenValidationFragment,
we're able to update the new ConfirmDeviceCredentialBaseFragment
that will handle results. We can also retain other important state like
the device credential guess to be set after successful validation.

Some smaller changes include:
* If the activity is finished for any reason other than "Back" getting
  pressed, RESULT_FIRST_USER is returned instead of RESULT_CANCELED.
* CheckBox, "Forgot [LSKF]?" button, and EditText/LockPatternView
  gets disabled during validation.
* The above also stay disabled if ConfirmDeviceCredentialBaseFragment
  gets re-created and remote lockscreen validation is still in progress.

Test: m RunSettingsRoboTests -j
ROBOTEST_FILTER=com.android.settings.password
Test: Manual
Bug: 274983372
Bug: 274991889
Bug: 274792310
Bug: 270395807

Change-Id: Ib6d47430e233a43e6985ab83abae45713c49771f