audiopolicy: clear caller identity before calling audio policy manager

Audio policy manager methods must always be executed with the
level of priviledge of audioserver process.
After checking calling permissions, audio policy service will
clear binder caller identity before calling into audio policy manager.

Also add restrictions for start source and stop source binder
transactions.

Bug: 77869640
Test: manual audio smoke tests
Change-Id: I993d4fddcea9bfa3f44093c38406b85d69ca8890