kernel: use sched lock for k_thread_suspend/resume

This logic should be using the sched_lock and not its own
separate lock for these two functions.

Some simplications were made; z_thread_single_resume and
z_thread_single_suspend were only used in one place, and there was
some redundant logic for whether to reschedule in the suspend case.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>