kernel: sched: Fix comparsion in _update_time_slice_before_swap()

The _update_time_slice_before_swap() function directly compared
_time_slice_duration (expressed in ms) with value returned by
_get_remaining_program_time() which used ticks as a time unit.

Moreover, the _time_slice_duration was also used as an argument
for _set_time(), which expects time expressed in ticks.

This commit ensures that the same unit (ticks) is used in
comparsion and timer adjustments.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>