Embms API adjustments for 7/21

* Enforce that only one instance of each manager can be active.
* Add a death receipient for both managers to notify the app of binder
death
* Add documentation informing the app that it may not call create()
multiple times
* Fix a collision in streaming state reason codes
* Add documentation in DownloadRequest to indicate which methods should
be called by the middleware.

Change-Id: Ie15283b5c34fee736e8023dbd4f889c2ca95299e