Add getObjectPropValue function to MtpDevice.

In the MTP spec, the object size is stored in MtpObjectInfo as unsigned
32-bit integer and fetched by the getObjectInfo operation. For the
objects that are more than 4GB, the object size is provided as one of
extra properties, which are fetched by different operation.

The CL adds to getObjectPropValue method to MtpDevice class so that
client code can obtain 4GB+ object size from object property.

BUG=27805369

Change-Id: I0b91facd07cdc19866cb29f7df08bb1698bcf60b