Fix memory leak in DRM framework

The listener member of a Drm object wasn't
being released, causing memory to be leaked
every time a MediaDrm instance was created.

bug:32277525
Change-Id: I5582056fff3d20a5fd2b178bf028008d7f9ad1b4
diff --git a/drm/libmediadrm/Drm.cpp b/drm/libmediadrm/Drm.cpp
index 9ab08db..07e9414 100644
--- a/drm/libmediadrm/Drm.cpp
+++ b/drm/libmediadrm/Drm.cpp
@@ -334,6 +334,7 @@
         return -EINVAL;
     }
 
+    setListener(NULL);
     delete mPlugin;
     mPlugin = NULL;