Fix intermittent crashes. Fixes T1227

Summary: Move pulling of post in the background from the onResume method. At times activity doesn't get attached before that method executes

Test Plan:
 - Launch app.
 - Exit app.
 - Launch again after some time.
 - App shouldn't crash as before.

Reviewers: dkobia

Subscribers: rjmackay, shadowhand

Maniphest Tasks: T1227

Differential Revision: https://phabricator.ushahidi.com/D567