Use pinned taskbar with desktop tasks on freeform displays

Bug: 390665752
Flag: com.android.window.flags.enable_desktop_taskbar_on_freeform_displays
Test: manual on desktop device. verify correct taskbar is shown
      on home screen, in overview, in desktop mode, with fullscreen
      app shown. Verify opening/launching apps from taskbar works
      as expected.

Change-Id: I5c1e21799609c28ec44cc190bfc681934907199f