Avoid selecting group item every time when groups are loaded

* Cause: when there are a great number of contacts in the a newly-added
  account, onGroupsLoaded will be called many times during contacts
  sync. As we set seletced menu item in it, the navigation drawer
  menu will jump again and again.
* Solution: we notify listener only when the loaded groups are changed.

Bug 30306059

Change-Id: I93c039861fbe74e672d95d03c4644b6d31b8bbe5