Allow automatic updates from 3rd party app stores

[grrrrr]

With 3rd party stores such as Droid-ify, Neo-Store and now F-droid, can all perform auto-updates of apps if the device is running Android 12 or newer. This means that the end user does not have to confirm an "install" prompt each time Osmtracker updates, it will instead of automatically installed as and when the store app checks for app updates.

I'd like to suggest that Osmtracker implements this feature now that it meets the minimum TargetSDK level of >=31.

Further details here:

https://developer.android.com/about/versions/12/features#automatic-app-updates