If a device gets added when we're already deactivated, plymouth shouldn't
process the device, since processing it effectively activates plymouth.

This commit pulls the udev monitor fd out of the event loop while
plymouth is deactivated so new events are deferred until reactivation.

Modified by Iain Lane <iain.lane@canonical.com>: Also deactivate the
timer that finds all devices known to udev after an interval, when
paused.