Disable default APN context when release network request is received.

Currently, default apn context remains in enabled state even after
receiving release network request from Connectivity Service.
This causes mobile internet data to get re enabled when WIFI is connected.
Explicitly disable default apn context when release network request is
received.

Change-Id: I9d81072d196939b1085dbd9a1b984a06b95566bb
CRs-Fixed: 746834