DISALLOW_CONFIG_MOBILE_NETWORKS for Mobile Network

 - Avoid intent start MobileNetworkSettings page
 - Avoid Mobile network preference show on screen

Flag: EXEMPT bug fix
Fix: 289232540
Test: Manual test. see b/289232540#28
Test: atest passed

Change-Id: I25b75673fbc0758dc06ca15f890e5dee0ea1367f