[flexiglass] Adds support for locking when entering dream state

Fix: 377335319
Test: manually verified that entering dream state locks the device with
the appropriate timeout and that exiting it before the timeout elapses
doesn't lock it
Test: unit tests added
Flag: com.android.systemui.scene_container

Change-Id: Ia52d4a67d799cb7375dbe7674ceb467983edcf17