This schedules CleanCacheService to run regularly, and delete files older
than the value set in the new "Keep cached apps" preference. It auto-
migrates the old "Cache packages" pref to the new one. The default cache
time for people who did not have "Cache packages" enabled is one day.
This makes it a lot easier to setup all the testing stuff. Mostly,
I'm tired of fighting Android Studio's fragility, so I want to remove
as much non-standardness as possible in the hopes of improving that
situation.
closes#534https://gitlab.com/fdroid/fdroidclient/issues/534