[Notifications] Competing notifications when two alarms kick at the same time

A cousin of bug #1354406

Some of the early testers are hitting a use case that we didn't account for, that of multiple alarms set for the same time.

Indicator-datetime currently tries to pull up notifications for, and play sounds for, both notifications. That's happening by accident and is clearly not the right approach.

I can think of a couple of approaches we might take:

-- SOLUTION --
If 2 or more alarms have been triggered, the notification at any time will display:
  Title: 2 Alarms
  Subtitle: [Label 1] 10:00, [Label 2] 10:01

The sound of the first alarm will be played. If they have the same time, the one that has been created first.

------ UX Comment ------

Use cases:

1 - User sets 2 alarms, at the same time, with same attributes (repeat, label, sound)

Expected behaviour: Clock app / alarm manager recognise a conflict of newly set alarm time with previously set alarm and rejects it if it’s found same.

2 - User sets 2 alarms, at the same time, with (one or more) different attributes (repeat, label, sound)

Expected behaviour: the alarm that was set in the first place is the one that goes off (with correspondent sound and label) and the second one is fired as a notification bubble.

3 - User sets 2 alarms, at different times, even if the difference is 1 minute. Then first alarm goes off and second alarm does it one minute after.

4 - Several notifications fired at the same time:

Desired behaviour described here: Page 35: Notifications queuing and flood prevention

https://docs.google.com/document/d/1xDSZ_dnAMAlhgFnnyjJEibaITXjVLp1_pnj_tATNm9I/edit#heading=h.5ldl02lx6rwi

"In general, applications can fire only one notification at a time, but multiple applications can fire their notification simultaneously. All notifications should be treated as if they are in a queue and will be handled sequentially according to their fire time (in a FIFO manner)."