Refactor code for storing preference for EventLogging opt-out

Right now, Funnel and MccMncStateHandler have to interact directly with the
SharedPreferences instance to get whether EventLogging is enabled or not. This
is quite clunky.

This patch abstracts this interaction to the WikipediaApp class, so that these
classes can use app.isEventLoggingEnabled() rather than interacting directly
with the SharedPreferences instance.

Change-Id: Ibba0e4baab923b4095cc8aa9f5a057f40f379977
Merge "Refactor code for storing preference for EventLogging opt-out"