Add a DummyInstaller to handle non apk files

Merged username-removed-1345 requested to merge cde/fdroidclient:nonapk into master
  • Avoids crashes when trying to treat non apk files, such as privileged extension ota as apks
  • Doesn't do anything at all for now.

One issue with this is the app is always in the not installed state, so what would be appropriate here would be to change the text of the install button to download, and update that accrodingly. However that is outside of the scope of this change.

TODO: Add toast indicating the user that they need to install manually, in case of OTA