Previously this only worked for apps which were being upgraded to the
latest available version. Now it works for apks being upgraded from an
old version to a newer-but-not-newest version. Perhaps more importantly,
it also now works for newly installed apps.
This has a problem where if a user installs, then uninstalls an app,
they are still notified about installing it. This is because the apk
is in the cache and the code doesn't know whether it is there from
the initial install, or because it has been redownloaded by the user
to install some time after uninstalling.
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