[audio] Add AudioManager getNativeInterface

Migrate to a sub-interface for native accessibly methods so we don't
have to manually parcel/transact new additions. The existing methods
will either be removed, or folded into the native interface via ongoing
refactoring.

Test: compiles
Bug: 376481063
Flag: EXEMPT mechanical refactor
Change-Id: Ief320a181c1c21076f1b1c8d1372808505bcd460