Make saving a contact less confusing

* Add always visible save menu item (a checkmark icon) on both
  the compact and full editors.  Selecting it saves changes
  and closes the editor (what back does currently).
* On the compact editor, make back perform a discard,
  including warning the user if there are unsaved changes.
* On the full editor, back still saves and goes back to
  the full editor. This is not ideal but it's out of
  scope to not save changes between the two editors
  when editing an existing contact.

Bug 19983789

Change-Id: Ie23b75978db70f2b438a824e39f0ae8fb8aa99ec