Remove jcenter() repository

JFrog announced the service end of the jcenter repository. It will stay in readonly mode, but no new versions will be deployed there.

See: https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

We should migrate away from it, when all dependencies that FFA uses are available elsewhere:

• PowerPreference
• Exoplayer