Set custom task transition spec when taskbar is visible

This is to ensure that the task to task animations look good when the
taskbar is visible (the transition background is the same as the taskbar
color, the taskbar overlay rounded corners are hidden, and the task
animates above the taskbar so it's rounded corners are visible during
the animation and not hidden behind the taskbar).

Test: Open tasks from other tasks to trigger task to task animations and
see if they look good
Bug: 200675009
Bug: 196387647

Change-Id: I8e46718829c0e6541a5c1b1a3a156285aeecd30a