albumartcache: Change idle priorities to low

Revert back to the low priority for all I/O calls as this should no
longer trigger too many open file descriptors with the delayed loading
of the covers patch.
3 jobs for wip/mschraal/albumsview-delayed-covers in 7 minutes and 45 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Check
passed #10033
pep8

00:02:45

passed #10032
pyflakes

00:02:43

 
  Build
passed #10034
build

00:04:59