Add WM transit types for dream activity.

This change adds two WM transit types for opening and closing dream
activities, which allows the system to define transition in and out of
dreams, that are currently being overriden by other transitions.

Set the priority to be just below keyguard transit. The keyguard
occlude/unocclude already correctly handles the dream open/close
animation.

Bug: 222507937
Bug: 220311554
Test: atest WmTests:AppTransitionControllerTest
Test: manually on device by entering and exiting dreams over keyguard,
launcher, and settings. observed animation and relevant types of WM logs.

Change-Id: Iedfa279d56cd9d4c4679cb3b697ee74a98ef727b