Fix ConcurrentModificationException in RotationChangeProvider

When bgHandler and CallbackHandler were different, ConcurrentModificationException could have happened.

Using a lock here is not ideal, as we would block {un}registration of listeners in the main thread.

Using a CopyOnWriteArrayList will fix the problem with a negligible memory overhead.

Fix: 344626706
Test: RotationChangeProviderTest
Flag: NONE bugfix
Change-Id: Ia2bf884e4f889b19ab85f0b7734509bbcd1f3b96