Persist settings on a dedicated background thread

Settings were persisted on the system background thread but during
first boot the device is under heavy load and persisting settings
competes with other system components using the shared background
thread. As a result persisting settings can be delayed much longer
than the expected 200ms. This can cause issues with setup wizard
being skipped/went over and its component disaabled being persisted
but the setting whether the device is provisioned not being
persisted - now if the device boots it will have no SUW but also
the home button would be missing. Generally, we need a tansactional
abstraction in the system process to peform all delayed operations
atomically.

bug:25472484

Change-Id: Icf38e72403b190a8fa9d0554b8dd83ce78da3bc8