Clean-up STLState.finishTransition()

This CL cleans up STLState.finishTransition() so that we don't have to
provide an idleScene anymore: it always finishes to the transition
current scene.

This also fixes a bug in STLState.snapToIdleIfClose: we should only snap
to fromScene (toScene) if progress is close to 0f (1f) AND currentScene
is equal to fromScene (toScene).

Bug: 353679003
Test: atest SceneTransitionLayoutStateTest
Flag: com.android.systemui.scene_container
Change-Id: Ic671bb7b396c9cfd9137213a0daca1797e41f812