Add ScrimShadeTransitionController back to ShadeStartable

It never should've been removed in the first place.
This reverts the changes to ShadeStartable that should
not have been merged as part of b/433477971.

Guard ScrimShadeTransitionController calls to
scrimController with sceneContainerFlag check.

Flag: EXEMPT bugfix
Test: atest ShadeStartableTest
Test: pull down notification shade - see scrim exists
Fixes: 441407223
Fixes: 433477971
Change-Id: Ibe2af48584decc0358ce14a50ce3b96fb39108f6