Hide DesktopTaskView background while animating

While animating DesktopTaskView to/from fullscreen, hide the background.
Background should only show up when this taskview is shown in recents.
While it is animating, we don't need to show it.

Bug: 263264985
Test: manual, trigger transient taskbar, observe that background does
not flicker

Change-Id: I76d0eb1857645b7f04a0b2e1eebe2393c252c257