Fix: update feedAdapter and reading lists from main thread only.
Fix: update feedAdapter and reading lists from main thread only.

This patch makes sure that we instantiate ReadingListPageObserver within
onCreate(), and give it a Handler that is tied to the UI thread of the
app, so that any onChange() callbacks that come from the observer happen
on the main thread.

Change-Id: I70be3de457995edef0e4db548a4a7b532dcd711f

wikimedia/apps-android-wikipedia,wikimedia/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,wikimedia/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,wikimedia/apps-android-wikipedia