Prevent multiple calls of ActivityThread.systemMain()

Bug: 170591440
Test: manually
Change-Id: Iba6d10e5203fa1c1f96c85e39a57483ee26e8111
(cherry picked from commit 8f2c59c0cb)

services/core/java/com/android/server/media/MediaShellCommand.java

@@ -64,7 +64,7 @@ public class MediaShellCommand extends ShellCommand {
         }
         if (sThread == null) {
             Looper.prepare();
-            sThread = ActivityThread.systemMain();
+            sThread = ActivityThread.currentActivityThread();
             Context context = sThread.getSystemContext();
             sMediaSessionManager =
                     (MediaSessionManager) context.getSystemService(Context.MEDIA_SESSION_SERVICE);