MediaRouter2: Add volume-related info to MediaRoute2Info

Prior to adding volume control functionaly to MediaRouter2,
this CL adds volume-related info to MediaRoute2Info.

From this, MediaRouter2 clients can show the current volume of the remote device.

Volume control logic will be followed by this CL.

Bug: 142298160
Test: atest mediaroutertest
Change-Id: I64a96db9c194e5a5d9aa3ce9b8792c0eb7f2eb4f