Workaround for sysui studio builds

- SysUI studio builds seem to have problems referencing the private
  attr directly, so just expose it as a color itself

Bug: 152086714
Test: Studio builds
Change-Id: I6256848ef7e608c7eb1773c83184219e8af47032

core/res/res/values/colors.xml

@@ -427,4 +427,7 @@
     <!-- Darkest shade of the secondary neutral color used by the system. Black.
      This value can be overlaid at runtime by OverlayManager RROs. -->
     <color name="system_neutral2_1000">#000000</color>
+
+    <!-- Exposes colorSurface in cases where it can't be directly referenced. -->
+    <color name="surface_above_background">?attr/colorSurface</color>
 </resources>

core/res/res/values/symbols.xml

@@ -4342,4 +4342,5 @@
   <java-symbol type="string" name="view_and_control_notification_content" />
 
   <java-symbol type="layout" name="notification_expand_button"/>
+  <java-symbol type="color" name="surface_above_background" />
 </resources>

packages/SystemUI/res-keyguard/values/styles.xml

@@ -17,7 +17,7 @@
 */
 -->
 
-<resources xmlns:androidprv="http://schemas.android.com/apk/prv/res/android">
+<resources>
     <!-- Keyguard PIN pad styles -->
     <style name="Keyguard.TextView" parent="@android:style/Widget.DeviceDefault.TextView">
         <item name="android:textSize">@dimen/kg_status_line_font_size</item>
@@ -32,7 +32,7 @@
         <item name="android:stateListAnimator">@null</item>
     </style>
     <style name="NumPadKey" parent="Theme.SystemUI">
-      <item name="android:colorControlNormal">?androidprv:attr/colorSurface</item>
+      <item name="android:colorControlNormal">@*android:color/surface_above_background</item>
       <item name="android:colorControlHighlight">?android:attr/colorAccent</item>
       <item name="android:background">@drawable/num_pad_key_background</item>
     </style>