Use date field if year/month/day is not set

[reox]

It looks like that many entries use the date field instead of year-month-day. The date field should have the same syntax as

library/src/com/cgogolin/library/BibtexEntry.java

Line 129 in 2686df0

public String getDateFormated() {

If the three separate fields are not set, all these entries are not sorted by date but added at the top.
It would be nice to have the fallback of date field, if no year field is set. JabRef handles this quite nicely.

I would try to fix it myself, but right now I'm fighting with the android sdk on debian ;)

[reox]

As ant seems to be pretty deprecated, I converted everything to gradle and also fixed this bug.
As I never did much Android programming, everything might be a little bit ugly...
But anyways, here is the stuff: https://github.com/reox/library/tree/gradlify

I tested it and I now have every entry with the correct year.
But I fear that all these changes need some care before they get merged.

Is anyone a proper Android app dev and might want to get over all the gradle stuff as well as the changes from support libs to androidx?

btw: I also run a code formatter over all the java files, just if anyone is wondering why the diff is so large. Maybe that wasnt the smartest idea though :D
I also fixed a few deprecation warnings and raised the SDK version.
I also tried to build it with the Debian Android SDK but had no success. There were a lot of weird error messages :(