Enable dream overlay on mobile with dreams v2 flag instead of hub v2

We've decoupled the two so use the new dreams_v2 flag to enable the
dream overlay on mobile.

Bug: 409218143
Test: atest DreamOverlayRegistrantTest
Flag: android.service.dreams.dreams_v2
Change-Id: Id6cdefddf6f52239b3e16b7829b9b283c3cd549a