Implement in-app updates

[jacqueline8711]

Issue

Fixes #1296

Not sure about whether to put it in MainActivity, StartupActivity or BaseActivity (at least the onResume to cover all entry points)
Not tested yet.

Screenshot

TODO when testing

https://developer.android.com/guide/app-bundle/in-app-updates

[kordianbruck]

So BaseActivity does not seem to fit as we want to be very predictable when such notifications happen. I'd also think that MainActivity is too late. Ideally we would show the update prompt, before any other requests/refreshes are started. It might be harder to implement it during startup, but it would save us some headaches. I'd also just do the immediate upgrades, so that people get the newest version right away.

Thoughts?

[thellmund]

What’s the state of this? @jacqueline8711’s implementation looks good to me (maybe encapsulate in a custom class) and I’m not sure I understand @kordianbruck’s concerns.

[jacqueline8711]

@thellmund I think my main problem was that I didn't know how to test if the code actually works since I didn't know how to trigger the in-app update / choose if the flexible or immediate update should be shown

[Bentipa]

What is the current state of this PR? @jacqueline8711