Preparation for adding the AddDesktopButton to the view hierarchy

- Update the logic of `getRunningTaskExpectedIndex` to make sure it
  will still work with the AddDesktopButton added as the first child
  of the RecentsView.
- Introduce `mTaskAlignmentTranslationY` to ClearAllButton, and set
  it through RecentsView. Thus, the logic can be used by the
  AddDesktopButton as well. etc.

Flag: EXEMPT refactor
Bug: 382057498
Test: Manual
Change-Id: I6d0c031fe1fc0b4c198b9c890a27a69d911d38d9