Make RouteSessionInfo#getId() return String rather than primitive int

This CL makes the session have String ID rather than
primitive integer ID.

Also, this CL makes RouteSessionInfo#getId() return a unique ID
when it is called for routes which were get via
MediaRouter2 / MediaRouter2Manager.

Bug: 147394941
Test: atest mediaroutertest
Change-Id: I715617243f1561cb00eb1cca6346ba772813516a