Make EMBMS adjustments for 08/04

* Add a download state callback to the callback formerly for progress
* Rename EXTRA_SERVICE_INFO to EXTRA_SERVICE_ID, and specify that it
should be a String rather than a ServiceInfo
* Add documentation to clarify that the progress/state callback may be
null

Bug: 30981736
Test: manual
Change-Id: Ie1ec99868f5b09a270b500dc97abeb9f819cfa56