Fix ConcurrentModificationException in FoldableDisplaySwitchTrackingInteractorTest

We're modifying callback list in UnfoldTransitionRepository while FakeUnfoldTransitionProvider is still iterating over them.
This exception is only thrown when running Robolectric tests as Robolectric's ArrayList implementation differs from Android's one.
While Android implementation will just stop iterating, Robolectric will continue and throw exception when trying to get `next()` element.

Solution is to either not modify the list from within callback method or modify it safely and I went for the later with CopyOnWriteArrayList.

Fixes: 412727194
Test: FoldableDisplaySwitchTrackingInteractorTest -  on device and deviceless
Flag: NONE just fixing test
Change-Id: I59558cd6a74a3afbdfaa7b8549bbc7083ae9c87d