Cache stream volumes to prevent ANR

MediaRourter uses main thread for internal operations, and ANR could
happens if AudioService is not running when MediaRouter tried to get
stream volumes. This CL avoids such ANR by caching stream volumes.

Bug: 170327593
Test: manually
(cherry picked from internal master branch)

Change-Id: I7c00b26a3a25f17c877d3ec0e998905ab0b4af28