transcoding: separate client interface from service

Refactor AIDL to separate client interface from service.

Use client listener binder as unique id for client, as multiple
clients could exist for the same pid.

Misc unit testing changes.

bug: 145233472
test: mediatranscodingservice_test, TranscodingClientManager_tests
Change-Id: I8d9522da23163375df8df7822d0f6ae270cff1b4