Add a flag to guard new MessageQueue test apis

This is to guard use of MessageQueue.peekWhen(), MessageQueue.pop() and
any other TestApi's we may need to make MessageQueue testable without
introspection.

Flag: android.os.message_queue_testability
Test: EXEMPT
Bug: 379472827
Change-Id: I4ef5b7dd3aa3d34971c6cc1a9d08075a71fdbd63