Remove using ServiceManager API in MediaSessionManager

MediaSessionManager calls ServiceManager#getService(string) to get
ISessionManager, but it is a hidden API that will not be made into
@SystemApi (ag/6329675).

Instead, create a wrapper class that can only retrieve specified
media service binders. (ag/10123575 for reference)

Bug: 159955826
Test: atest
    CtsMediaTestCases:android.media.cts.MediaSessionManagerTest
Change-Id: I1ab89d1217e8e3d071b38da772ee96889e9859b1