image cache clean-up to clean swap disk

Currently swap backing files are never removed from the system.
The image clean-up code should recognise at least obsolete unused files
need to be removed.

Change-Id: I09519a73c7233f2dc6f46f6cd094a3e1cc3dc395
Partial-Bug: 1253991a
Merge "image cache clean-up to clean swap disk"