Add human readable dump of volume handling to routing session

This CL reuses code in MR2Info to provide a human readable string for
volume adjustment in MR2 info, and in the toString implementations of
both MR2Info and RoutingSessionInfo.

Test: mp core services && adb shell dumpsys media_router
Bug: 205124386
Flag: NONE Minor changes in debug-related logging.
Change-Id: Id3194d3c7f6eded5c37e2d64b43989de80ed1765