Add a class for managing Session priority

Priority is given first to the system priorty session, then to
active local sessions, then to active remote sessions, then to
the rest of the sessions. Ordering within categories is by whoever
last performed an action we associate with the user.

The stack has methods for getting filtered sets of this priority.

This also:
-Changes publish to setActive(boolean)
-Adds a flag for handling media buttons.
-Adds a flag for transport controls instead of enabling once.
-Unhides the setFlags API.
-Updates the legacy helper to use the flags.

Change-Id: I6ebeb27410de1b24149fd6e1785613ac444f0774