Home default setting should not shown in managed profile settings

Each default preference needs to have its corresponding
PreferenceAvailabilityProvider to provide us the availability of it.
If no corresponding provider is found, it is considered to be not
available. So it encourages other developers who will add new default app
preference later to consider the availability of it.

Bug:27143673
Change-Id: I073b7122dddc579504f397c5de2bdd4df3826269

src/com/android/settings/applications/DefaultBrowserPreference.java

@@ -25,6 +25,7 @@ import android.os.UserHandle;
 import android.util.AttributeSet;
 
 import com.android.settings.AppListPreference;
+import com.android.settings.PreferenceAvailabilityProvider;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -72,4 +73,11 @@ public class DefaultBrowserPreference extends AppListPreference {
 
         return result;
     }
+
+    public static class AvailabilityProvider implements PreferenceAvailabilityProvider {
+        @Override
+        public boolean isAvailable(Context context) {
+            return true;
+        }
+    }
 }