[INTERNAL] overworked ContentProvider

To avoid massive code duplication the layout of the content providers is
changed to provide better chaining of different content providers.

Also moved some model classes to different packages and implemented some
Roster counterparts for MDNS.

Change-Id: Iae95721405ceddc21ded417115bdbd2466194bb2
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1553
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>