alarm: generate alarm_id in API

This isn't a detail of the storage, this is something we handle and
expose in the API, so the responsability for handling should be in the
API.
Also, they shouldn't exist any alarm without an id, so the model makes
it mandatory now.

Change-Id: I726ca74489007ee1d387bcb06b23af2f060ceb5f
Merge "alarm: generate alarm_id in API"