Mms auto-retrieval (1/2): framework

- Settings: Add auto-retrieval and auto-retrieval-on-roaming strings
- GsmDataConnectionTracker: Gets settings and auto-connects APN_TYPE_MMS

Patch set 2   : Fix NPE when the preference was not yet set to a defaulted
                value
              : Add @hide's
              : Set proper defaults

Change-Id: I9c4ea47cd39cd017fba3d5b0649d0e84aaf78b4e