[flexiglass] validateSceneChange returns false when from == to

There's no need to do any work if a scene change is called for but the
destination scene is the same as the current scene.

Flag: com.android.systemui.scene_container
Test: manually changed scenes on the device
Test: unit tests added
Bug: 395063194
Change-Id: I912c957d8bc7a2c136ad52c55320087259c95e98