Xabber: use jitpack instead of untrusted igniterealtime repo.

Xabber upstream uses an untrusted Maven repository (igniterealtime) to download the latest versions of Smack. I have replaced this with jitpack.io which imports Smack from igniterealtime's github and creates Maven packages. Updates now need to be manual to ensure the versions used by Xabber match the commit used by jitpack. A gradle.properties entry has been added to make this easy, so only one line of the Xabber metadata needs changing for each update.

I have removed the auto-update flags. When a new version of Xabber is released, the commit hash of the version of smack used needs to be updated to match the version used upstream. I am happy to do this.