task_wdt: Fix the way the kernel timer is used

Do not use periodic executions of the timer handler, as in certain
circumstances (the fallback hardware watchdog used, one or more
task_wdt channel activated but none of them being ever fed) this
would lead to no callback/reset being executed for any channel.
Instead, schedule the next timeout from the timer handler function
when the function is executed for the dummy background channel or
for a channel that was deleted.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>