ShadeLockscreenInteractor: change scene on main thread

If we do this on the background thread, we trip a new assert in
SceneTransitionLayout, so do it on the main thread instead.

(This already landed once as ag/27496336, but was accidentally undone in
ag/27607465; I took the opportunity to make it clearer this time.)

Fixes: 347269674
Test: manual: set lockscreen, post notif, go to lockscreen, expand notif
Flag: com.android.systemui.scene_container
Change-Id: I6fc7c55f1db564db60ca55de2974286ed3bc7f60