Add OnComputeInsetsListener to TaskbarContainerView

Instead of updating visibility of TaskbarContainerView, keep it
VISIBLE but with alpha 0, and update touchableInsets to allow
touches to pass through. This avoids sending insets changed
when Taskbar is hidden.

Test: Swipe to Overview, no jumping/jank due to insets change;
also tap where Taskbar would be and ensure it doesn't launch

Bug: 171917176
Change-Id: I9ccb4335e0301f34eec459657f3bbaf88b0d8a52