Fix issue with stack order positioning when ending recents animation.

- When moving the home stack from a higher index to behind a stack with a
  lower index, the position does not need to be adjusted in order to move
  the stack (since positionChildAt first removes the stack before adding
  it). Updated the existing tests with a case to move the stack to a
  non-bottom index to verify this.

Bug: 73901604
Test: atest ActivityStackTests

Change-Id: If1d3af0f35d60569d8a3d86898483bacaaa98bb9