Show unsynced groups clearly in separate item, don't annoy.

This change adds a new "More groups..." list item under the
expandable list for a data source when it has any unsynced
groups, making it more discoverable for users to bring back
unsynced groups.  This fixes http://b/2084771

Also cleans up lingering issues so we show unsynced groups
from the correct accounts, and only prompt the user once
about removing "ungrouped."  Fixes http://b/2114723