Contacts: Do not observer DB change in MemberListActivity when it paused

While import SIM contacts in background, switching among PeopleActivity,
MemberListActivity and Luncher could cause lots of DB operations and data
change notifications. As a result, the screen will be frozen or dark
during these operations.

Do not observer DB change in MemberListActivity when it paused, since it
will requery in onResume.

CRs-Fixed: 659023

Change-Id: I02866b262c86a9dab4ecda5b7138020b2d035d5c