Dispatching the `MediaDataEncoder::Encode()` invocation is not really
    necessary because the callback is desinated to run in task queue anyway.
    Also make `Shutdown()` wait for completion to make sure resource is
    released before next allocation.

Includes parts from:
* Bug 1662514 - Take mCallbackMutex when accessing mError.
* Bug 1662540: Remove unused member.
* Bug 1640416 - p5: run WebrtcMediaDataEncoder and the platform encoder in different thread pools/task queues.