Fixing new shell keyguard inconsistency

The change fixed the following two issues:
1. Keyguard going away should matches the keyguard shown logic - no new
   transition should be created by the server if the new keyguard
   transition is enabled;
2. Updating the corresponding test to match the updated logic.

Test: DisplayContentTests
Bug: 364930619
Flag: com.android.window.flags.ensure_keyguard_does_transition_starting
Change-Id: Ifee7d3d229106e14b2a5b23c8da1803eb9821022