Reset factory of TaskPositioner in test

Depending on the execution order of test methods, after executing
TaskPositionerTests#testOverrideFactory, the next test method in
TaskPositioningControllerTests may be failed because the test
static factory returns null.

Clear the factory so it will use default implementation that returns
non-null TaskPositioner.

Bug: 254268165
Test: atest TaskPositionerTests TaskPositioningControllerTests
Change-Id: I6c9abb60ce6d1a8631ad07b63d6e0d04c2f76b39

services/tests/wmtests/src/com/android/server/wm/TaskPositionerTests.java

@@ -88,7 +88,7 @@ public class TaskPositionerTests extends WindowTestsBase {
 
     @After
     public void tearDown() {
-        mPositioner = null;
+        TaskPositioner.setFactory(null);
     }
 
     @Test