Fix restoring network mode preference on reboot

When a user sets the option to use only 2G networks (for GSM networks) or its
equivalent for CDMA networks, each reboot erases that setting. That forces
the user to re-select the same option after each boot.
This (very simple) patch fixes it.

Ported from CM9:
http://review.cyanogenmod.org/#/c/16245/

Change-Id: I7ec9e155fed9f57934d7a73936f2bf425c8297fe