Add API to set a default session in Activity

We may have multiple remote sessions at once so an app needs a way
to specify a default session to route media and volume commands to
when its activity is in the foreground. This adds an api that works
in parallel with the default stream api on Activity.

bug:15727927
Change-Id: I5b67cb3e20647f9437d90aa65be63efe8c7865cf