Add home screen risk cell update button countdown

[johannesrohwer]

Description

This PR implements a countdown for the update button that is shown for the risk cell on the home screen. (see JIRA 1626)

Checklist

• This PR does not change text that hasn't been signed off with the RKI. Have a look at the pinned issue 440
• Test your changes as thoroughly as possible before you commit them. Preferably, automate your test by unit/integration tests.
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[haosap]

I see, the timer invalidate is only called in the CoutdownTimer class:

 deinit {
		timer?.invalidate()
		timer = nil
	} 

My question is, if the app goes background, the Timer still runs? What will happened if the user opens the App after 2 days?

[johannesrohwer]

thanks for bringing this up, I discovered a bug in my implementation where the timer was not stopped as I expected due to a reference cycle. I added a manual invalidate() method to make sure the timer is handled correctly.
Regarding the opening of the app after some time has passed: The timer is guaranteed to be up-to-date since the scene delegate will trigger the updateExposureState() method of the home VC when the app comes into foreground.
To make sure the timer does not drain the users battery when the app goes into background, I added a few lines that invalidate the handler when the app goes into background.

[melloskitten]

I have added some comments and cleaned up some of the code. Would be cool if you looked through these commits, and checked whether they make sense to you. Thanks! 👍

In case there aren't enough tests: Please make sure to add tests, cheers! 😄

[haosap]

Well done!💯