Telephony: Use proper method to read prefered network

getIntAtIndex from MSimTelephonyManager should be used
to read preferred network type. Current implementation
is not using proper method and hence failing, resulting in
network preferred mode changing unwantedly in UI.
Also modified code accordingly as Preferred network mode is moved
from Secure to Global.
CRs-Fixed: 511081

Change-Id: Id44ba283302451e485ce8bef1d6d8bd60d10c6e8