Fix race while setting ASyncNotify callback

Holding mAsyncCallbackLock while setCallback() in
mediacodec can cause deadlock. setCallback() is done without lock
but the ASyncCallback and userdata assignments are done within
the lock so that we can respond to async callback immediatly

bug: 236908959
Change-Id: Id0d2befa8520cbd35b6265037e6481276dcb1bfc