Adding API for getting the root node info in the active window.

1. This APIs is useful for accessibility services since very often
   they want to get access to the root node in the current window
   to determine where accessibility focus is and subsequently find
   a next view to take accessibility focus.

Change-Id: Ide5231f832d9ce2e9b14dc411dfd675f753a4edb

api/current.txt

@@ -1994,6 +1994,7 @@ package android.accessibilityservice {
 
   public abstract class AccessibilityService extends android.app.Service {
     ctor public AccessibilityService();
+    method public android.view.accessibility.AccessibilityNodeInfo getRootInActiveWindow();
     method public final android.accessibilityservice.AccessibilityServiceInfo getServiceInfo();
     method public abstract void onAccessibilityEvent(android.view.accessibility.AccessibilityEvent);
     method public final android.os.IBinder onBind(android.content.Intent);

core/java/android/accessibilityservice/AccessibilityService.java

@@ -426,13 +426,11 @@ public abstract class AccessibilityService extends Service {
             throw new IllegalStateException("AccessibilityService not connected."
                     + " Did you receive a call of onServiceConnected()?");
         }
-        AccessibilityNodeInfo root = AccessibilityInteractionClient.getInstance()
-                .findAccessibilityNodeInfoByAccessibilityId(connectionId,
-                        AccessibilityNodeInfo.ACTIVE_WINDOW_ID, AccessibilityNodeInfo.ROOT_NODE_ID,
-                        AccessibilityNodeInfo.FLAG_PREFETCH_DESCENDANTS);
+        AccessibilityNodeInfo root = getRootInActiveWindow();
         if (root == null) {
             return;
         }
+
         AccessibilityNodeInfo current = root.findFocus(AccessibilityNodeInfo.FOCUS_ACCESSIBILITY);
         if (current == null) {
             current = root;
@@ -479,6 +477,19 @@ public abstract class AccessibilityService extends Service {
         }
     }
 
+    /**
+     * Gets the root node in the currently active window if this service
+     * can retrieve window content.
+     *
+     * @return The root node if this service can retrieve window content.
+     */
+    public AccessibilityNodeInfo getRootInActiveWindow() {
+        return AccessibilityInteractionClient.getInstance()
+            .findAccessibilityNodeInfoByAccessibilityId(mConnectionId,
+                AccessibilityNodeInfo.ACTIVE_WINDOW_ID, AccessibilityNodeInfo.ROOT_NODE_ID,
+                AccessibilityNodeInfo.FLAG_PREFETCH_DESCENDANTS);
+    }
+
     /**
      * Performs a global action. Such an action can be performed
      * at any moment regardless of the current application or user