Add APIs for creating a system priority session and getting controllers

This adds a hidden call to set flags and a flag for making a session an
exclusive high priority session. This will cause all media button events
to be sent to that session as long as it is stillr egistered. This
requires the MODIFY_PHONE_STATE permission like the old forCalls API.

This also adds a way to get controllers for all the ongoing sessions.
This is protected by the MEDIA_CONTENT_CONTROL permission like the
old RemoteController APIs.

Change-Id: I51540e8dcf3a7dbe02a0f8ee003821e40af653a3