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 the Xabber metadata needs changing for each update.