Add a test api, TaskInfo#getDisplayId().

- Some CTS needs to filter out Tasks in non-default display.

Bug: 235576683
Test: atest android.devicepolicy.cts.LockTaskTest
Change-Id: I2faf700fbe18ac8548dd4d405bc1b8092a4db2ff

core/api/test-current.txt

@@ -419,6 +419,7 @@ package android.app {
   public class TaskInfo {
     method public boolean containsLaunchCookie(@NonNull android.os.IBinder);
     method @NonNull public android.content.res.Configuration getConfiguration();
+    method public int getDisplayId();
     method public int getParentTaskId();
     method @Nullable public android.app.PictureInPictureParams getPictureInPictureParams();
     method @NonNull public android.window.WindowContainerToken getToken();

core/java/android/app/TaskInfo.java

@@ -426,6 +426,15 @@ public class TaskInfo {
         return parentTaskId != INVALID_TASK_ID;
     }
 
+    /**
+     * @return The id of the display this task is associated with.
+     * @hide
+     */
+    @TestApi
+    public int getDisplayId() {
+        return displayId;
+    }
+
     /**
      * Returns {@code true} if the parameters that are important for task organizers are equal
      * between this {@link TaskInfo} and {@param that}.