MediaSessionService: Fix NPE when startup

Bug: 158050954
Test: Manually tested that the issue is gone after following command
      $ adb shell
      $ settings secure put media_button_receiver ",10"
      $ reboot
Change-Id: I0d7df2d349140c09282d029df80c2cf2060c5405

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

@@ -85,6 +85,9 @@ final class MediaButtonReceiverHolder {
             return null;
         }
         ComponentName componentName = ComponentName.unflattenFromString(tokens[0]);
+        if (componentName == null) {
+            return null;
+        }
         int userId = Integer.parseInt(tokens[1]);
         // Guess component type if the OS version is updated from the older version.
         int componentType = (tokens.length == 3)