InAppNotification doesn't wait to display all messages in stack/queue mode

[michael-hawker]

Describe the bug

When using the InAppNotification control with a StackMode of QueueBehind or StackInFront we don't wait long enough to show each message for the notification duration time before dismissing them.

• Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

Steps to Reproduce

Steps to reproduce the behavior:

1. Go to the InAppNotification sample in the Sample App
2. Set the Notification Duration to 1000
3. Set StackMode to either QueueBehind or StackInFront
4. Click on the 'Show notification with random text' button a number of times.

Note that the display time of each message seems inconsistent (especially with shorter duration values and smaller numbers of messages, and I think more so with QueueBehind).

Expected behavior

Each message to be displayed for the notification duration time before the next one is shown.

Environment

NuGet Package(s): 

Package Version(s): 

Windows 10 Build Number:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] May 2020 Update (19041)
- [ ] Insider Build (build number: )

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [ ] Insider Build (xxxxx)

Device form factor:
- [x] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio 
- [ ] 2017 (version: )
- [ ] 2019 (version: ) 
- [ ] 2019 Preview (version: )

Additional context

Add any other context about the problem here.

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