MediaRouter2: Refactor internal classes

This CL introduces following changes:
 - Remove ambiguous 'client' word usages
 - Clarify AIDL file names and related classes/methods/fields
 - Reorder AIDL methods

Bug: 149642195
Test: CTS and atest mediaroutertest
Change-Id: I53656c3be52ab8deb3fbf96bf7caf9a3476d7af6