Add more unit tests for mobile data preferred uids feature

- Add more tests for mobile data preferred uids.
- Also address leftover comments on implementation commit.

Bug: 171872461
Test: atest FrameworksNetTests
Merged-In: I63f65bffc1b8883ddfd774222e26e629d7ea1f94

Change-Id: I63f65bffc1b8883ddfd774222e26e629d7ea1f94

service/src/com/android/server/ConnectivityService.java

@@ -1516,7 +1516,7 @@ public class ConnectivityService extends IConnectivityManager.Stub
     }
 
     // Note that registering observer for setting do not get initial callback when registering,
-    // callers might have self-initialization to update status if need.
+    // callers must fetch the initial value of the setting themselves if needed.
     private void registerSettingsCallbacks() {
         // Watch for global HTTP proxy changes.
         mSettingsObserver.observe(