Make PreferenceController a mixin

Bug: 62912136
Test: Existing tests in BaseSearchIndexProviderTest
Change-Id: Ieda359806c09a019840b2005446c7ec8b61fdb00
diff --git a/src/com/android/settings/deviceinfo/FeedbackPreferenceController.java b/src/com/android/settings/deviceinfo/FeedbackPreferenceController.java
index ed61a92..2d8941d 100644
--- a/src/com/android/settings/deviceinfo/FeedbackPreferenceController.java
+++ b/src/com/android/settings/deviceinfo/FeedbackPreferenceController.java
@@ -21,10 +21,12 @@
 import android.support.v7.preference.Preference;
 import android.text.TextUtils;
 
-import com.android.settings.core.PreferenceController;
+import com.android.settings.core.PreferenceControllerMixin;
 import com.android.settingslib.DeviceInfoUtils;
+import com.android.settingslib.core.AbstractPreferenceController;
 
-public class FeedbackPreferenceController extends PreferenceController {
+public class FeedbackPreferenceController extends AbstractPreferenceController implements
+        PreferenceControllerMixin {
     private static final String KEY_DEVICE_FEEDBACK = "device_feedback";
 
     private final Fragment mHost;