Use isAbleToDream for DREAM->OCCLUDED transition

This signal is more stable than the isDreaming signal, and seems to
remove the race condition where we may erronously go to OCCLUDED first
before going to LOCKSCREEN when terminating dream.

Fixes: 411095671
Test: atest FromDreamingTransitionInteractorTest
Test: manually on device by entering and exiting dreams while occluded
and not occluded
Flag: EXEMPT bugfix

Change-Id: I7ecefa775a4f26b57a27f25c39456e75c5ed674d