Text size for empty view shall be styled according to theme

In activity_list.xml the empty TextView has set a
textAppearance which allows the theme to set the font size.
However, for preference_list_fragment.xml the textAppearance
is missing but should follow the same style as
activity_list.xml

The missing textAppearance makes the empty text in
Settings->Wifi and Settings->Bluetooth look small if
no networks are found.

Change-Id: I9320b37cfeec0d9505d73650696eaf534fc4a321

core/res/res/layout/preference_list_fragment.xml

@@ -41,6 +41,7 @@
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:padding="@dimen/preference_fragment_padding_side"
+        android:textAppearance="?android:attr/textAppearanceMedium"
         android:gravity="center"
         android:visibility="gone" />