[Settings] Code refactor

1. Fix comments
2. Split test case

Bug: 147338143
Test: make RunSettingsRoboTests ROBOTEST_FILTER=DataUsageListTest
Change-Id: I017a3ac8a716e69eb4d9c3e30e20363a215563a2
diff --git a/src/com/android/settings/datausage/BillingCyclePreference.java b/src/com/android/settings/datausage/BillingCyclePreference.java
index 8dea7de..47ef56b 100644
--- a/src/com/android/settings/datausage/BillingCyclePreference.java
+++ b/src/com/android/settings/datausage/BillingCyclePreference.java
@@ -96,7 +96,7 @@
     }
 
     /**
-     * Implementation of MobileDataEnabledListener.Client
+     * Implementation of {@code MobileDataEnabledListener.Client}
      */
     public void onMobileDataEnabledChange() {
         updateEnabled();