MediaRouter2: add MRM.getActiveRoutes

This method will list up routes that are actively used
by some applications.

Once dynamic grouping is implemented, it will return
sessions instead of routes.

Bug: 146604082
Test: atest mediaroutertest

Change-Id: Ic013d3c2ded47b7ebca0b4cf99d2a7ea75045dc1
2 files changed