Use correct IDs when collecting multipart config parts

Change Id23749e672fd4154e3d9986e22b46fc038579d8e caused a regression
where the multipart resource cannot fetch the configs for the specified
parts. This change fixes that regression.

Change-Id: Ic42565e35f96e1b249cba4682eef34cd0ec694ce
Closes-Bug: #1395902
Merge "Use correct IDs when collecting multipart config parts"