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
11 files changed