pm: policy: default: Optimize power state selection

Update power state selection. Previously, it was iterating over states
starting from the last one so the most common short sleep periods were
taking the longest time to select. Order is now swapped so that short
sleeps will get power state as quick as possible.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>