Subtract taskbar size from DeviceProfile#availableHeight

Normally availableHeight excludes system insets, but taskbar is added
after availableHeight is calculated. This discrepency causes some bad
calculations such as TaskView size.

Test: touch nav bar when in an app with taskbar present, ensure the
task doesn't jump due to incorrect calculation

Fixes: 179478728
Bug: 171917176
Change-Id: I16d39d3e1d9708e2a64c81b1ac24466f9567d1b9