Add TransitionLink feature

Transitions can now be linked. E.g. when one STL transitions A->B then
the linked STL should automatically play C->D. The configuration can
be passed to SceneTransitionLayoutState via `transitionLinks`.

Test: SceneTransitionLayoutStateTest
Bug: b/320257219
Flag: NONE
Change-Id: I6192ddf489f1d2fd895b20af3b4428aa6e18fa32