libmediaplayerservice: fix warnings, make warnings errors, use clang

Change-Id: I1b2f6b65c5abbc366068a60b8909104f31b94228
diff --git a/media/libmediaplayerservice/Drm.h b/media/libmediaplayerservice/Drm.h
index 1591738..c4013b8 100644
--- a/media/libmediaplayerservice/Drm.h
+++ b/media/libmediaplayerservice/Drm.h
@@ -26,8 +26,8 @@
 
 namespace android {
 
-struct DrmFactory;
-struct DrmPlugin;
+class DrmFactory;
+class DrmPlugin;
 struct DrmSessionClientInterface;
 
 struct Drm : public BnDrm,