[59/n] Create TransitionObserverTestContext and clean up

Creates the TransitionObserverTestContext to simplify testing
for all TransitionObserver implementation.

Also removes duplicate code.

Flag: EXEMPT Test only
Bug: 419766870
Bug: 375339716
Test: atest WMShellUnitTests:DelegateLetterboxTransitionObserverTest
Test: atest WMShellUnitTests:LetterboxLifecycleControllerImplTest

Change-Id: Ia0f168b6806df2637af6888c917d80746ecdd832