Fix some change-transition issues and tests

Moved some freeze teardown to WindowContainer.

Only perform transition on leaf-task.

Now that display is changed before configuration update
after reparent, updated a test to ensure the old display
doesn't get the transition.

Removed Flaky annotation since these shouldn't be flaky.

Bug: 153705881
Test: atest AppChangeTransitionTests
Change-Id: Ic8f66bba553942f6bccc04121054a568de2ea592