Clean up streaming API docs and organize errors

* Implement listPendingDownloads and resetDownloadKnowledge
* Bring MbmsStreamingManager docs in line with vendor-side API docs.
* Remove activeStreamingServicesUpdated
* Reorganize the errors in MbmsException into categories.
* Add some vendor API docs for file-download

Change-Id: Ia29066ea6f2c0414171cffa1ae11bb467f187bee