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