Fix a JNI local reference leak in MediaDrm

A reference leak was found in the subtitle usecase that also
applies to android_media_MediaDrm.cpp
(Code taken from the similar fix in android_media_MediaPlayer.cpp)

Change-Id: Icc4b25e4adc9a600ff4ac99a7a8478bb55a6e348
diff --git a/media/jni/android_media_MediaDrm.cpp b/media/jni/android_media_MediaDrm.cpp
index 6d90ccb..ca26e4a 100644
--- a/media/jni/android_media_MediaDrm.cpp
+++ b/media/jni/android_media_MediaDrm.cpp
@@ -186,6 +186,7 @@
             nativeParcel->setData(obj->data(), obj->dataSize());
             env->CallStaticVoidMethod(mClass, gFields.post_event, mObject,
                     jeventType, extra, jParcel);
+            env->DeleteLocalRef(jParcel);
         }
     }