Properly constrain leanback preference icon size

Change-Id: I181eea8515378ead7eb38e39943a75a43dc33bfc
diff --git a/v17/preference-leanback/res/layout/leanback_preference.xml b/v17/preference-leanback/res/layout/leanback_preference.xml
index 498b107..05ca21c 100644
--- a/v17/preference-leanback/res/layout/leanback_preference.xml
+++ b/v17/preference-leanback/res/layout/leanback_preference.xml
@@ -34,8 +34,8 @@
         android:layout_gravity="center_vertical" >
         <ImageView
             android:id="@android:id/icon"
-            android:layout_width="wrap_content"
-            android:layout_height="wrap_content"
+            android:layout_width="@dimen/lb_preference_item_icon_size"
+            android:layout_height="@dimen/lb_preference_item_icon_size"
             android:layout_marginEnd="@dimen/lb_preference_item_icon_margin_end"
             />
     </FrameLayout>