pm: policy: event: use uptime ticks

Update events to use uptime ticks, which is a monotonic clock which
in the same res as kernel ticks. This makes comparisons simple and
removes the complexity of dealing with wrapping counter values.

The wrapping is particularly problematic for events since this makes
it quite complex to track if an event has occured in the past, or
will occur in the future. This info is needed to know if an event
has actually been handled or not.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>