Make ISessionManager.getSessions() return list of tokens

This CL changes the return type of ISessionManager.getSessions()
from List<ControllerLink> to List<MediaSession.Token>.

Bug: 123488663
Test: Ran below tests
atest CtsMediaTestCases:android.media.cts.MediaSessionTest;
atest CtsMediaTestCases:android.media.cts.MediaControllerTest;
atest CtsMediaTestCases:android.media.cts.MediaSessionManagerTest;
Change-Id: Ie4d259dc3691977451c7751ae06816d67079a3f4