[Large Screen] Launch MobileNetworkActivity as a fragment for split mode

Receive intent in MobileNetworkActivity and launch a activity which extend SettingsActivity

Test: atest
Bug: 197609014
Bug: 209606859
Change-Id: Ic07b18b3ea11744f240b25a0986474df763a199b

src/com/android/settings/Settings.java

@@ -304,6 +304,8 @@ public class Settings extends SettingsActivity {
     public static class WifiCallingDisclaimerActivity extends SettingsActivity { /* empty */ }
     public static class MobileNetworkListActivity extends SettingsActivity {}
     public static class PowerMenuSettingsActivity extends SettingsActivity {}
+    public static class SubscriptionSettingsActivity extends SettingsActivity {  /* empty */  }
+
     /**
      * Activity for BugReportHandlerPicker.
      */