Show downloaded + not installed apps in "Updates"

This is one of the last major things I wanted to fix before focusing on bug fixes and then stable release.

It was really annoying that when you:

  • Initiate a download+install,
  • Wait until after downloading (but before installing),
  • You'd correctly (and very helpfully) see the apk waiting in "Updates", and accompanied by a red badge on the Updates tab,
  • If you leave it (don't install it), close F-Droid, and return later,
  • It would no longer be there ready for you to install, even though the .apk file is in downloaded and in the cache.

Now it will correctly notify you about these apks. In the process, it also fixed a bug where if you upgraded an apk from version X (old version) to version Y (newer, but not the newest/suggested version), it now correctly should show in "Updates".

Fixes #962 (closed).