build: Added settings to allow pushing to maven central.

[BeksOmega]

I added some settings and played with some gradle things so that hopefully this can be published on maven central.

I did this by following the following two tutorials:

• https://android.jlelse.eu/the-complete-guide-to-creating-an-android-library-46628b7fc879
• https://medium.com/@zubairehman.work/a-complete-guide-to-create-and-publish-an-android-library-to-maven-central-6eef186a42f5

They agree on most things, so hopefully that means we are good.

The sonatype issue is here: https://issues.sonatype.org/browse/OSSRH-53565