Fix logging spam in DashboardFragment.

When updating preferences managed through PreferenceController, the
fragment should skip prefs that are not available.

Bug: 32255863
Test: RunSettingsRoboTests
Change-Id: I2f9b6ddf8c78d40068dc18f07e60672dcba4474a

src/com/android/settings/network/ProxyPreferenceController.java

@@ -36,7 +36,7 @@ public class ProxyPreferenceController extends PreferenceController {
     }
 
     @Override
-    protected boolean isAvailable() {
+    public boolean isAvailable() {
         // proxy UI disabled until we have better app support
         return false;
     }