drm: use DrmSessionManager for session resource managing.

Bug: 19265536
Change-Id: Ie35184f99c51e7bd94d7c826ffb8e89040a81905
diff --git a/media/libmediaplayerservice/DrmSessionManager.h b/media/libmediaplayerservice/DrmSessionManager.h
index 1d0ed43..ba5c268 100644
--- a/media/libmediaplayerservice/DrmSessionManager.h
+++ b/media/libmediaplayerservice/DrmSessionManager.h
@@ -42,6 +42,8 @@
 typedef KeyedVector<int, SessionInfos > PidSessionInfosMap;
 
 struct DrmSessionManager : public RefBase {
+    static sp<DrmSessionManager> Instance();
+
     DrmSessionManager();
     DrmSessionManager(sp<ProcessInfoInterface> processInfo);