Don't remove finished transitions early when other transitions run

This CL changes STLState so that transitions running at the same time
are all removed only when all transitions are finished. This was a small
optimization that can make some scrollable elements starting a second
transition disappear, which can lead to unnecessary cancellations.

Bug: 378470603
Test: atest SceneTransitionLayoutStateTest
Flag: com.android.systemui.scene_container
Change-Id: I18e15f012b4caa03b557038c166c54ea358d028a