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/EncryptionInterstitial.java

@@ -92,7 +92,7 @@ public class EncryptionInterstitial extends SettingsActivity {
         private int mRequestedPasswordQuality;
 
         @Override
-        protected int getMetricsCategory() {
+        public int getMetricsCategory() {
             return MetricsEvent.ENCRYPTION;
         }