Clean up visibility related flags in WindowToken

- Replace isHidden() and setHidden() by isVisible() and setVisible()
  in accordance with variable names.
- Move isVisible() from AppRecord to WindowToken

Test: Refactoring CL, existing tests pass.
Bug: 80414790
Change-Id: Ie77bf1d00336e655b1fbee15d419c6959da2e0c9