MTP: Partial implementation of the GetObjectPropList command

In this initial implementation we only support fetching one property at a time.
Support depth = 0 (single object) or depth = 1 (all objects in a directory)
Reimplemented GetObjectPropValue on top of GetObjectPropList, since the former
is a special case of the latter.

Change-Id: Ia76ee61741d6ee3902b5c5d9fc094cf86dfaf650
Signed-off-by: Mike Lockwood <lockwood@google.com>
diff --git a/media/mtp/MtpProperty.cpp b/media/mtp/MtpProperty.cpp
index f7c12d6..86889c3 100644
--- a/media/mtp/MtpProperty.cpp
+++ b/media/mtp/MtpProperty.cpp
@@ -53,7 +53,7 @@
         mDefaultArrayValues(NULL),
         mCurrentArrayLength(0),
         mCurrentArrayValues(NULL),
-        mGroupCode(0),
+        mGroupCode(-1), // disable multiple properties in GetObjectPropList for now
         mFormFlag(kFormNone),
         mEnumLength(0),
         mEnumValues(NULL)