kernel: Update CPU runtime stats of non-idle time

Updates sched_cpu_update_usage() such that the CPU runtime stats
only update the its non-idle time when the current thread is not the
idle thread. This is necessary as otherwise the CPUs idle-time will
be double counted in k_thread_runtime_stats.execution_cycles.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>