Bug 1036861 - [MTP] Write file size into the response of getObjectInfo, r=dhylands

In the response of operation GetObjectInfo, there is a field
"ObjectCompressedSize". Currently it's assigned with 0. We need
to put correct file size into it. I also corrected the type of
property value in function getObjectPropertyDesc().
Bug 1036863 - [MTP] Rename file/folder via MTP, r=dhylands

This patch fixed 'unable to rename file/folder via MTP' by
responding to MTP operation 'setObjectPropertyValue' correctly