STL avoid multiple onStop calls

Due to NestedScrollDispatcher, the order of method calls (pre/post
scroll/fling) cannot be guaranteed and they could be called multiple
times.

Test: atest PriorityNestedScrollConnectionTest
Bug: 377666808
Flag: com.android.systemui.scene_container
Change-Id: Ic8334e37b90f99d583c9bb79a13d636661810b2a