This also converts old Repo.lastUpdated values rather than just failing.
index.xml handling used to store the Repo "Last Updated" date used to store
the value as just an ISO date (2019-04-29), then the time was added. So if
date/time parsing fails, this falls back to trying to parse just the date.
null is returned when parsing fails, and the Latest Tab shows nothing if
the Last Updated is null.
Some related tests were also tweaked.
Hopefully:
closesfdroid/fdroidclient#1757
c0c5721f6a