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>