Only use DefaultMixedHandler for recents when desktop tasks are visible

We were always using DefaultMixedHandler for recents transition when
desktop feature flag was enabled. This is not required and we only need
to use it when tasks are actually visible.
When they are not, leave recents transition handling for the
RecentsTransitionHandler.

Bug: 299679976
Test: Open apps on desktop, swipe up to recents, observe from logs that
      DefaultMixedHandler is the transition handler for recents.
      Open an app in fullscreen, swipe up to recents, observer from logs
      that RecentsTransitionHandler is the transition handler for
      recents.
Change-Id: I047de19b9f92366b876c50713995373c4ac9f4a8