Move task positioning tasks from WindowManagerService.mH

The CL moves task positioning tasks from WindowManagerService.mH to
TaskPositioningController.mHandler.

Bug: 64149458
Test: atest com.android.server.wm.TaskPositioningControllerTests
Change-Id: I2a8ce22a1698e008b2abd2e8f1b06886cec697b4
diff --git a/services/core/java/com/android/server/wm/TaskPositioner.java b/services/core/java/com/android/server/wm/TaskPositioner.java
index ca9f3a9..fa7ea2f 100644
--- a/services/core/java/com/android/server/wm/TaskPositioner.java
+++ b/services/core/java/com/android/server/wm/TaskPositioner.java
@@ -203,7 +203,7 @@
 
                     // Post back to WM to handle clean-ups. We still need the input
                     // event handler for the last finishInputEvent()!
-                    mService.mH.sendEmptyMessage(H.FINISH_TASK_POSITIONING);
+                    mService.mTaskPositioningController.finishTaskPositioning();
                 }
                 handled = true;
             } catch (Exception e) {