Enable support for groups in External contacts accounts

This permits use with DavDroid and other external providers, who are unfairly
disadvantaged by Google, most likely to try to favour their own closed contact
syncing services. This levels the playing field, and lets external contacts
providers also do groups (as there's nothing otherwise stopping them)

Change-Id: I6b8133c03a1a647e52b32007a4891051c2c57999