Changes

GMM: rework PDP context status

Use an uint16 instead of an pointer with an unknown length.
Also encode the current active PDP context (e.g. on 2G/4G
mobilty this could change). It resynchronizes the UE with the SGSN
state.

Change-Id: I12d3110c9365132f96ef7ff8a1be22a431682c81