Improve interruptions of NestedSTLs

Before this CL there were situations where the parent state was set to
null which caused shared elements to not animate at all after e.g.
reversing a nested transition and then starting a parent transition.

Instead of relying on attach/detach we now monitor local transitions
and make sure to assign the parent state as sensible as possible.

An even better solution where interruptions would never cause jump cuts
would require a more sophisticated solution, see code comment.

Bug: 376659778
Test: manual tests in demo app
Flag: com.android.systemui.scene_container
Change-Id: I4ff107395573662cd43ed7e62a29f40152144fd9