Add an activity that starts DataUsageList fragment for mobile.

We are providing an itent and a corresponding activity to show
"Mobile data usage" fragment, so that "Mobile network setting"
who lives in phone apk can use that intent to open it.

Bug: 62201918
Test: Manual
Change-Id: I862f8bc90f9456f91b436d26472cce23c837a777
diff --git a/src/com/android/settings/Settings.java b/src/com/android/settings/Settings.java
index ce0f838..94602d9 100644
--- a/src/com/android/settings/Settings.java
+++ b/src/com/android/settings/Settings.java
@@ -90,6 +90,7 @@
     public static class CryptKeeperSettingsActivity extends SettingsActivity { /* empty */ }
     public static class DeviceAdminSettingsActivity extends SettingsActivity { /* empty */ }
     public static class DataUsageSummaryActivity extends SettingsActivity { /* empty */ }
+    public static class MobileDataUsageListActivity extends SettingsActivity { /* empty */ }
     public static class ConfigureWifiSettingsActivity extends SettingsActivity { /* empty */ }
     public static class SavedAccessPointsSettingsActivity extends SettingsActivity { /* empty */ }
     public static class TextToSpeechSettingsActivity extends SettingsActivity { /* empty */ }