Only compute drag distance when necessary

This CL is a small refactor in SceneGestureHandler so that the drag
distance and target Scene are computed only when the target SceneKey has
changed.

Bug: 291071158
Test: PlatformComposeSceneTransitionLayoutTests
Flag: N/A
Change-Id: I57f2090bcfc711bcb3b9bd220d86b4cc0bf54a86
1 file changed