MediaSessionLegacyHelper uses global Application context

MediaSessionLegacyHelper used a Context passed by an application
  which can cause a leak of Activity instances. Use the global
  Application context instead.
Also prevent crash if a null Context is supplied (method not
  documented as requesting a non-null Context).

Bug 18767503

Change-Id: I8281047d0af233a323f3fc11ababedff848829ec

media/java/android/media/session/MediaSessionLegacyHelper.java

@@ -69,12 +69,9 @@ private MediaSessionLegacyHelper(Context context) {
     }
 
     public static MediaSessionLegacyHelper getHelper(Context context) {
-        if (DEBUG) {
-            Log.d(TAG, "Attempting to get helper with context " + context);
-        }
         synchronized (sLock) {
             if (sInstance == null) {
-                sInstance = new MediaSessionLegacyHelper(context);
+                sInstance = new MediaSessionLegacyHelper(context.getApplicationContext());
             }
         }
         return sInstance;