Mark #testStartAndFinishPositioning as flaky

This test has been flaky for a while producing a NPE when
trying to register a TaskPositioner.

TaskPositioner is legacy code that is no longer used anywhere,
so just mark it as FlakyTest to unblock presubmit until the
legacy code is removed.

Bug: 291067614
Test: atest WmTests:TaskPositioningControllerTests
Change-Id: Id5b4b8666ad2fb10828493d3930f28ac009a1371

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

@@ -34,6 +34,7 @@ import static org.junit.Assert.assertTrue;
 import android.platform.test.annotations.Presubmit;
 import android.view.InputChannel;
 
+import androidx.test.filters.FlakyTest;
 import androidx.test.filters.SmallTest;
 
 import org.junit.Before;
@@ -72,6 +73,7 @@ public class TaskPositioningControllerTests extends WindowTestsBase {
         doReturn(mock(InputMonitor.class)).when(mDisplayContent).getInputMonitor();
     }
 
+    @FlakyTest(bugId = 291067614)
     @Test
     public void testStartAndFinishPositioning() {
         assertFalse(mTarget.isPositioningLocked());