Refactor SwipeTransition to have immutable from and to Scene

SwipeTransition could change from and to during an ongoing transition.
e.g. BOUNCER -> LOCKSCREEN, then LOCKSCREEN -> SHADE. This makes it hard
for observers of the TransitionState to notice when a transition was
updated. Now the TransitionState has to be updated with a new transition
when to or from have changed.

Test: new and old tests in SceneGestureHandlerTest
Bug: b/310915136
Flag: NONE
Change-Id: If4c1a61eb1b92e85bd9f054d404b9dc4b312d781