Add hidden setter for AppWidgetHost's InteractionHandler

This will allow Quickstep launchers, e.g. NexusLauncher, to
provide a custom interaction handler.

Bug: 169042867
Test: manual testing

Change-Id: Id70cb463e2671e32ea9f52f06487ac63fcf57e0d
diff --git a/core/java/android/appwidget/AppWidgetHost.java b/core/java/android/appwidget/AppWidgetHost.java
index a72877e..fe81df0 100644
--- a/core/java/android/appwidget/AppWidgetHost.java
+++ b/core/java/android/appwidget/AppWidgetHost.java
@@ -320,6 +320,15 @@
     }
 
     /**
+     * Set the host's interaction handler.
+     *
+     * @hide
+     */
+    public void setInteractionHandler(InteractionHandler interactionHandler) {
+        mInteractionHandler = interactionHandler;
+    }
+
+    /**
      * Gets a list of all the appWidgetIds that are bound to the current host
      */
     public int[] getAppWidgetIds() {