Prevents users from being added if device is not provisioned

Also added a utility function for checking provisioning, instead of
repeating code for this check in several places.

bug:26156445
Change-Id: I9f4a280dd0cdf889f892e386dbf6a3fdb2a052ef

src/com/android/settings/DeviceInfoSettings.java

@@ -231,8 +231,7 @@ public class DeviceInfoSettings extends SettingsPreferenceFragment implements In
             if (!mUm.isAdminUser()) return true;
 
             // Don't enable developer options until device has been provisioned
-            if (Settings.Global.getInt(getActivity().getContentResolver(),
-                    Settings.Global.DEVICE_PROVISIONED, 0) == 0) {
+            if (!Utils.isDeviceProvisioned(getActivity())) {
                 return true;
             }