Fix null-pointers in SurfaceTransactionApplier constructor.

A recurring class of null pointers are caused by using views with no view root implementation in SurfaceTransactionApplier from QuickstepTransitionManager. This can happen when we use launcher views after it has been destroyed.

- No longer using mDragLayer in getFallbackClosingWindowAnimators; simply applying the transaction immediately.
- Forcefully using getFallbackClosingWindowAnimators when launcher is destroyed.

Flag: not needed
Bug: 278833389
Test: launches and closed several apps in 3-button and gesture nav mode
Change-Id: I83b3aec1488fe9666bd0301a6044a181bb05dbdd