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
diff --git a/src/com/android/settings/TestingSettings.java b/src/com/android/settings/TestingSettings.java
index 264db52..0286da7 100644
--- a/src/com/android/settings/TestingSettings.java
+++ b/src/com/android/settings/TestingSettings.java
@@ -39,7 +39,7 @@
     }
 
     @Override
-    protected int getMetricsCategory() {
+    public int getMetricsCategory() {
         return MetricsEvent.TESTING;
     }
 }