Avoid AAudioService->AAudioServiceInterface inheritence

This is required in order to prepare for divergence of
AAudioServiceInterface from BnAAudioService: if AAudioService
inherits from both, we would have method collisions as soon as we
change the signatures on one of the interfaces.

We achieve the same behavior with an adapter, which in this change is
completely trivial, but will be used as soon as the interfaces
actually diverge.

Bug: 160253486
Test: Manual testing using OboeTester
      Ran atest CtsNativeMediaAAudioTestCases
Change-Id: I0d3e41e2378f1e6ff9cbe37461682f7373f463f2