Incremental work on the group edit flow

* Extract group metadata class from members list fragment
* Move metadata loading up to the Activity so it can be
  shared between the members list and editor fragments
* Move options menu creation in the members list fragment
  up to the Activity and remove the obsolete members list
  callbacks (also rename the members list callbacks
  interface to "Listener" to match the rest of Contacts).
* Use a custom ViewPager that disabled swipes to move
  between the members list and editor fragments (also added
  a TODO to rename GroupMembersListActivity since it houses
  the editor now too).
* Update the group editor fragment to have group metadata
  passed in as an argument rather than loading it internally
  since we need it for the group members list view and it's
  already being loaded by the group members activity.

Bug 28707265
Bug 18641067

Change-Id: If662abd720533827e3e719fde4dd10cdfb1ba64e