Make MediaRoute2Info#getId() unique in client side (MR2/Manager)

This CL makes MediaRoute2Info#getId() return a unique ID when it is
called for routes which were get via MediaRouter2 / MediaRouter2Manager.

Acordingly, this CL removes MediaRoute2Info#getUniqueId().

Bug: 147338138
Test: atest mediaroutertest
Change-Id: Id2960c45e9cabde7bc5b57554608aebe03fcbe04