Rename Stack to Task related terms in tests

Rename Stack to Task and unify some methods to make them clearer and
simpler.

Bug: 144248496
Test: atest AppTransitionControllerTest AppTransitionTests
      atest DisplayContentTests DragDropControllerTests
      atest RecentsAnimationControllerTest RootTaskTests
      atest TaskDisplayAreaTests TaskLaunchParamsModifierTests
      atest TaskTests TransitionTests WindowContainerTests
      atest WindowManagerServiceTests WindowOrganizerTests WindowStateTests
Change-Id: Ifc7e4d86f3b19dc55a25293d65ae53856e93bf4d