Remove flicker from Communal widget launches.

Note that this is a sub-optimal implementation, as it relies on
scheduling the scene change with the animation duration instea of just
doing it in the moment when the animation ends. However, due to a bug
with STL, this flicker is currently inevitable if we rely on the end of
animation callback.

Bug: 333717642
Flag: com.android.systemui.communal_hub
Test: atest LegacyActivityStarterInternalImplTest CommunalTransitionAnimatorController
Change-Id: I2c664879f371afddc116a2d0baa5bc30b1222233