Fix SceneGestureHandler edge cases

1. if both upOrLeft and downOrRight become `null` during a transition
this will crash
2. if one of them changes during a transition, the transition will jump
cut to the new target

Test: new and old tests in SceneGestureHandlerTest
Bug: b/310915136
Flag: NONE
Change-Id: Ic4527a0e13b949116ac2bd805302419b6b8b5d47
2 files changed