Include previous state in alarm notification

We now provide both the current and previous alarm states
in the notification.

This allows differential logic to be applied by the webhook
implementation, for example depending on whether the previous
state was known or insufficient data.

It also allows the initial and subsequent notifications to be
distinguished for repeat_actions alarms, without resorting to
fragile string comparisons between 'Transition to ...' and
'Remaining as ...'.

Change-Id: I61294e98ddf504b3ab22e9b16ab718d64c27486f
Merge "Include previous state in alarm notification"