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.

Change-Id: Ib9e4bba6da705c127ce1f3aa3c242b9262541cee