Improve alarm validation

[WesleyBranton]

I strongly dislike the workaround implemented to correct inconsistencies with the Alarms API. To avoid missed alarms, the add-on runs an alarm check every time the browser detects a change in any tab. This works, but fires way too often.

In an ideal world, we could trust that the Alarms API will fire when it's supposed to and would not need validation at all. Sadly, history has proven this will not be the case.

However, there has to be a better approach than spamming the validation check tens of times per minute.