Don't start the random harvester process until timers are working.

This is a no-op currently, but in kernels with earlier AP startup, the
random kthread was trying to use timeouts with sleeps before timers are
working. Wait until SI_SUB_KICK_SCHEDULER to start the random kproc.

Reviewed by: delphij, imp, markm
Approved by: so
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D5712