storaged: handle negative delta in io usage

When UID is uninstalled and reinstalled, IO usage of the UID is
reset. This caused problem in accounting since it assumes IO usage
is always increasing for the same UID. In such case, use the current
usage instead of negative delta for accounting.

Bug: 34198239
Change-Id: I74661feccc05d19ba8a0feff8a2e38d72c7d1465