Update TCP buffer size when switching network type

Import of non-merging change 41612 from AOSP

When PDP connects in GSM network, the TCP buffer window size is
set to max 8760. If the phone swicthes to UMTS, the TCP buffer
size is not updated to UMTS, i.e. GPRS buffer size is still
used with limited data transfer spead as a result.
This fix makes sure the TCP buffer size is updated when
switching network type.

Change-Id: Ic44a557677ffe629e83ce1f522a9f058bee6e7b2