Use a content observer for getting the default APN

Instead of executing a query of the database for the
default APN on every network state change, an observer
is used on the database to be notifited when the apn
has changed. This prevents unneccesary queries on the
the database.

CRs-fixed: 662137

Change-Id: I3155bd8b53a1877413ed54a861df1bfb1853b9e4