fix: Show previously downloaded APKs in source selector dialog

[Aunali321]

What it does: Shows previously downloaded APKs in the "Change source" dialog. Closes #2535

Test steps:

1. Install the APK
2. Select an app to patch (e.g. YouTube), let it download the APK via a downloader
3. Patch and install it (so the download is saved in the DB)
4. Go to patch the same app again → tap "Select APK source"
5. In the dialog, between "Installed" and the downloader entries, you should see a "Downloaded" entry with the version number
6. Tap it. It should select that downloaded APK as the source
7. If there's a required version and the downloaded version doesn't match, the entry should appear greyed out

[oSumAtrIX]

"Downloaded" could be confusing "Already downloaded" or something similar would be better. Also maybe it should show up at the top of the list. Auto should also select an already downloaded apk instead of just auto choose one of the downloaders and thus the subtitle should be changed to accomodate that.

[Aunali321]

@oSumAtrIX

[Aunali321]

Auto already prefers a previously downloaded APK via MainViewModel.findDownloadedApp (PR #2967)