Cache cleaner wrongly deletes cache for non invalid images

Based on current implement of method delete_stalled_files,
it's just deleting all the images located in /incomplete
instead of using the param 'older_than' to check if it
should be deleted. So this fix will use 'older_than' to
confirm brfore deleting.

Fixes bug 1228256

Change-Id: Ica27915f8b9b098ecae1e38c8e678d6acf4f2f76
Merge "Cache cleaner wrongly deletes cache for non invalid images"