Introduce SessionCallbackLink

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

Also, this CL renames ControllerCallbackLink.asBinder() to
ControllerCallbackLink.getBinder().

Bug: 121239657
Test: make update-api -j / make -j
      atest CtsMediaTestCases:android.media.cts.MediaSessionTest
      atest CtsMediaTestCases:android.media.cts.MediaControllerTest
Change-Id: Icbcc4b045b1f6933c63d0d87aedc23b28c03d2de