Consolidate the two group metadata classes

* group.GroupMetadata is renamed to GroupMetaData

* The defaultGroup and favorites fields from
  common.GroupMetaData are added to group.GroupMetaData.

Test: manual tests
  - Add and edit a group
  - Load quick contacts w/ and w/o groups
  - Add invisible and directory contacts to default group

Change-Id: I7f53ae6e4409c414918ffc5615fb5e3bd25d116d