Introduce SessionCallbackLink

In order to avoid direct AIDL usages in framework, this CL introduces a
new public class SessionCallbackLink which can prevent usages of
ISessionCallback.

Bug: 122336924
Test: make update-api -j / make -j
      atest CtsMediaTestCases:android.media.cts.MediaSessionTest
      atest CtsMediaTestCases:android.media.cts.MediaControllerTest
Change-Id: I185d372335104067b6f4ed512fcd2f3b3925642f