F-Droid: Implement auto updates

[jakubgs]

We have successfully deployed two versions of the Android app to F-Droid as part of #8512.

Right now the update process is manual and described in the doc/FDROID.md file. But it is possible to automate this process using the AutoUpdateMode, UpdateCheckMode, and UpdateCheckIgnore settings:

https://f-droid.org/en/docs/Build_Metadata_Reference/#AutoUpdateMode
https://f-droid.org/en/docs/Build_Metadata_Reference/#UpdateCheckMode
https://f-droid.org/en/docs/Build_Metadata_Reference/#UpdateCheckIgnore

The most sensible solution would be to use the AutoUpdateMode: Version and/or UpdateCheckMode: Tags setting.

The main problem to solve is how to match the versionCode with the one for the Play Store release. This is important in order to avoid F-Droid release being able to be installed on top of the Play Store one or vice versa depending on order.

As @lokalise-bot commented below, the versionCode issue might not be as serious.

[licaon-kter]

You can't install since Android doesn't allow this, different singnature.

[licaon-kter]

That's me the bot :)

Anyway, the versionName is more important as people tend to compare that between stores and then open issues with "when is this going to be updated too"

versionCode is usually not that visible.

[cammellos]

Converting to a discussion since it looks like it's not yet actionable and we need a bit more discussion to specify the work, feel free to reopen if necessary