Remove starting window in shell transition.

This CL allows earlier removal of the starting window, even before the
transition finishes. Rather than waiting for state collection or the
core to receive the transaction commit callback, the shell monitors the
state and removes the starting window when both conditions are matched.
This avoids waiting for the Window Manager's global lock, which results
in earlier removal.

Flag: com.android.window.flags.remove_starting_in_transition
Bug: 397353053
Bug: 310046998
Test: atest StartingWindowControllerTests
Test: verify the starting window can be removed for cold and hot
launch, also verify it can be removed after transfer starting window.
Test: close activity/task with back gesture, verify the snapshot
surface can be removed eailer before transition finish.

Change-Id: I3dd55e8c68b99d02a49ee002e3109dc8c03122d8