Ensure that transitions are started only once

This CL ensures that a single transition can not be started multiple
times.

This uncovered a bug introduced in ag/29024667: linked transitions would
run() their originalTransition rather than simply awaiting for them.
This later unveiled that we were using an incorrect coroutineScope {}
for linked transitions, as we would wait for all linked transitions to
finish before actually re-running the transition again in the original
STLState.

I didn't add more tests for linked transitions because I plan to remove
them in ag/30320655 anyways.

Bug: 376438969
Test: atest SceneTransitionLayoutStateTest
Flag: com.android.systemui.scene_container
Change-Id: Ie91771bea7e2d1db869b302305093283a2604943