Fix the return type of MtpDevice#getStorageID().

Both MtpObjectHandle and MtpStorageID are typedef of uint32_t, but
MtpStorageID is more appropriate semantically.

BUG=None

Change-Id: Ibf5bb2105c54f7b73df559132d8737c272a1cefa
diff --git a/media/mtp/MtpDevice.h b/media/mtp/MtpDevice.h
index edc608f..970a58f 100644
--- a/media/mtp/MtpDevice.h
+++ b/media/mtp/MtpDevice.h
@@ -100,7 +100,7 @@
     bool                    sendObject(MtpObjectHandle handle, int size, int srcFD);
     bool                    deleteObject(MtpObjectHandle handle);
     MtpObjectHandle         getParent(MtpObjectHandle handle);
-    MtpObjectHandle         getStorageID(MtpObjectHandle handle);
+    MtpStorageID            getStorageID(MtpObjectHandle handle);
 
     MtpObjectPropertyList*  getObjectPropsSupported(MtpObjectFormat format);