Add ability to show/hide System Update.

This adds one flag:
config_show_system_update_settings

Which when set to false will hide "System update" from System page. This
is useful for devices that controls its system update elsewhere.

Bug: 69813881
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=com.android.settings.deviceinfo
Change-Id: Ibd1291ffd8948419e8aa06d0721842246a069378

src/com/android/settings/deviceinfo/SystemUpdatePreferenceController.java

@@ -47,7 +47,8 @@ public class SystemUpdatePreferenceController extends BasePreferenceController {
 
     @Override
     public int getAvailabilityStatus() {
-        return mUm.isAdminUser()
+        return mContext.getResources().getBoolean(R.bool.config_show_system_update_settings)
+                && mUm.isAdminUser()
                 ? AVAILABLE
                 : DISABLED_UNSUPPORTED;
     }