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
Ignore-AOSP-First: Needs cherry-picks
Change-Id: I63f65bffc1b8883ddfd774222e26e629d7ea1f94
diff --git a/service/src/com/android/server/ConnectivityService.java b/service/src/com/android/server/ConnectivityService.java
index b4355b9..0d0ed17 100644
--- a/service/src/com/android/server/ConnectivityService.java
+++ b/service/src/com/android/server/ConnectivityService.java
@@ -1512,7 +1512,7 @@
     }
 
     // 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(