Fix update crash in case of unset dates in APK; add an option to use APK's entry date

I encountered an error when I tried to add TrackID APK (files in the APK don't have dates set): as far as I can remember datetime(*manifest.date_time) was raising ValueError.

Also add an option to use date from APK as this APK's entry date (may be useful in case of a simple binary repo): fdroid update --use-date-from-apk.

I should have split this commit, but I hope this would be OK too.