Fix race condition in binder thread

Changing wallpapers too quickly could cause a race condition where
RemoteCallbackList#beginBroadcast would be called twice without
the corresponding call to end the broadcast.

Change-Id: I2e63df69ff4ffde76a0b91a2ad46f03d044d95b7
Test: Set breakpoint to pause thread and cause race condition.
Bug: 64391687