Define ICON_UNAVAILABLE for no icon status

Bug: 147776885
Test: make -j42 RunSettingsRoboTests
Change-Id: I0fd0e203bb7590955d0caf2679ebc78be68b4e50

src/com/android/settings/panel/PanelContent.java

@@ -28,11 +28,13 @@ import java.util.List;
  */
 public interface PanelContent extends Instrumentable {
 
+    int ICON_UNAVAILABLE = -1;
+
     /**
      * @return a icon resource for the title of the Panel.
      */
     default int getIcon() {
-        return -1;
+        return ICON_UNAVAILABLE;
     }
 
     /**