Late initialize TransitionViewModels in tests

All transitionViewModels that are underTest should
either be initialized with lateinit (or lazily).

Annotated flags are applied after the test class
initialization, so we don't want to initialize the
underTest before flags are applied, else we can
get into an inconsistent state.

Flag: EXEMPT test updates
Test: atest (all files updated)
Bug: 409851925
Change-Id: I46036113015e40157049aacc70bdb24f55e12acc