Fix Observation feed to handle filter changes
Fix Observation feed to handle filter changes

* Now that we only start the FRC for the observation feed once in viewDidLoad we need to
handle filter changes in the nsuserdefaults callback to update the predicate.  For this
to work we must ensure we don't remove the nsuserdefaults listener when leaving the view.

ngageoint/mage-ios,ngageoint/mage-ios,ngageoint/mage-ios,ngageoint/mage-ios