Settings: Add enable/disable 4G settings.

Add enable/disable 4G settings in settings root screen. When
check/uncheck it, set a default 4G/3G network mode

CRs-Fixed: 676565

Conflicts:
	res/values-zh-rCN/strings.xml
	res/values/bools.xml
	src/com/android/settings/SettingsActivity.java

Change-Id: Ice2c70f8f22253adee4a5574fd797d89dff65572