Make Network & internet v2 feature flag be persistent

In ag/5386198 we added a flag to turn on the new features of the network
and internet settings page, but that flag resets to the default value
(currently false) every time you reboot, which is annoying and hostile
for dogfooders. This CL makes the flag persistent.

Bug: 116349402
Test: manual / should be covered by existing tests
Change-Id: I098073e81feb199751e967a9652bec6a6445b95e

src/com/android/settings/development/featureflags/FeatureFlagPersistent.java

@@ -37,6 +37,7 @@ public class FeatureFlagPersistent {
     static {
         PERSISTENT_FLAGS = new HashSet<>();
         PERSISTENT_FLAGS.add(FeatureFlags.HEARING_AID_SETTINGS);
+        PERSISTENT_FLAGS.add(FeatureFlags.NETWORK_INTERNET_V2);
     }
 
     public static boolean isEnabled(Context context, String feature) {