ionice the update engine daemon to idle priority.
ionice the update engine daemon to idle priority.

This is to ensure that updates don't impact system responsiveness.

BUG=4592
TEST=tried updates with idle priority while browsing;
also tried running update in parallel with 10ms
write/flush/sync in parallel with tail -f -- worked OK

Review URL: http://codereview.chromium.org/3037042

joshfire/chromiumos-platform-init