Allow keyguard transition handler to play going away animation...

...even when systemui is launch activity over keyguard.
Althought the ActivityStarterInternal would wrap the launch activity
animation to remote transition, there can still have chance to unlock
keyguard just after start activity request.
In core, there can have two scenario:
1. Core could send two separate transition to shell, first one is the
   open app transition and should be handled by remote transition. The
   second transition is keyguard going away, which should be handled by
   KeyguardTransitionHandler.
2. Core only send one transition which contains KeyguardGoingAway and
   Occluding flag. To ensure the status correct in Keyguard, this
   transition should be handled by KeyguardTransitionHandler.
Occluding state can be correct through PhoneWindowManager ->
KeyguardService, so just need to make sure the going-away signal has
send to SystemUI.

Flag: EXEMPT bugfix
Bug: 352053143
Test: launch activity over keyguard, then dismiss the app, verify nothing
is break.

Change-Id: I8254ffe93547dd24e7835d846994d0c940bbea62