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