MTP: Fix reading and writing property descriptors

We were missing support for the group code, resulting in an off by 4 bytes problem.

Change-Id: I4a640c67b8d36e1bd658b6185178a71966d4b0b6
Signed-off-by: Mike Lockwood <lockwood@android.com>
diff --git a/media/mtp/MtpProperty.h b/media/mtp/MtpProperty.h
index 4923d40..c5b4e28 100644
--- a/media/mtp/MtpProperty.h
+++ b/media/mtp/MtpProperty.h
@@ -42,6 +42,8 @@
         kFormRange = 1,
         kFormEnum = 2,
     };
+
+    uint32_t            mGroupCode;
     uint8_t             mFormFlag;
 
     // for range form