Log visibility change for all fragments.

Bug: 30681771
Test: SettingsUnitTests

Refactor visibility logging from InstrumentedFragment into a mixin. And
apply mixin in remaining fragments.

Change-Id: Ibbb59904336254a3e4bb9e8c7d0b36e5a6bc2622

src/com/android/settings/wifi/SavedAccessPointsWifiSettings.java

@@ -59,7 +59,7 @@ public class SavedAccessPointsWifiSettings extends SettingsPreferenceFragment
     private static final String SAVE_DIALOG_ACCESS_POINT_STATE = "wifi_ap_state";
 
     @Override
-    protected int getMetricsCategory() {
+    public int getMetricsCategory() {
         return MetricsEvent.WIFI_SAVED_ACCESS_POINTS;
     }