Makes AccessibilityNodeInfo#getSourceNodeId as test API

For testing the accessibilityNodeInfo constructor methods, making this
method, AccessibilityNodeInfo#getSourceNodeId, to become a testing API.

Bug: 154163930
Bug: 154163953
Test: a11y CTS & unit tests
Change-Id: I9a2ff665996e28d6d5770723d98b49f96eab1f50

api/test-current.txt

@@ -4984,6 +4984,7 @@ package android.view.accessibility {
 
   public class AccessibilityNodeInfo implements android.os.Parcelable {
     method public void addChild(@NonNull android.os.IBinder);
+    method public long getSourceNodeId();
     method public void setLeashedParent(@Nullable android.os.IBinder, int);
     method public static void setNumInstancesInUseCounter(java.util.concurrent.atomic.AtomicInteger);
     method public void writeToParcelNoRecycle(android.os.Parcel, int);

core/java/android/view/accessibility/AccessibilityNodeInfo.java

@@ -3437,6 +3437,7 @@ public class AccessibilityNodeInfo implements Parcelable {
      * @hide
      */
     @UnsupportedAppUsage
+    @TestApi
     public long getSourceNodeId() {
         return mSourceNodeId;
     }