AppDetails2: bring back getInstalledApk() from AppDetails

Merged username-removed-1345 requested to merge cde/fdroidclient:new-ui/getInstalledApk into master
  • Addition: Try to get apk details via InstalledAppProvider too.
  • In certain cases, such as the "UnifiedNlp (no GAPPS)" app on a device with actualy GAPPS / GMS installed, apk can be null which leads to a crash
  • Ask InstalledAppProvider for the app's details too, like it was done in the old UX AppDetails.
  • Also seen when uninstalling app with a signing key different, fixes #985 (closed)