Use the new isNetworkSupported api for wifi-only

Didn't have an API for this before so people used a hacked system property (ro.carrier)
to determine if the device supported mobile data.  Added new API and switching callsites.

bug:5087537
Change-Id: Ibd799559be102a9e2fd552d1a23d1afbcf8f4614

src/com/android/settings/WirelessSettings.java

@@ -129,7 +129,7 @@ public class WirelessSettings extends SettingsPreferenceFragment {
         }
 
         // Remove Mobile Network Settings if it's a wifi-only device.
-        if (Utils.isWifiOnly()) {
+        if (Utils.isWifiOnly(getActivity())) {
             getPreferenceScreen().removePreference(findPreference(KEY_MOBILE_NETWORK_SETTINGS));
         }