Fix jank resulting from TaskView resizing

Don't include taskbar height when calculating TaskView size, otherwise
this results in an unnecessary jump when the taskbar changes, either
when navigating from DesktopWindowing to Overview or pinning the
Taskbar in Overview

Bug: 343882478
Fix: 373319262
Test: unbundled/launcher/nexus_image_test_platform OverviewImageNoTransientTaskbarTest
Flag: NONE Bugfix
Change-Id: I09de0e8b4386dd9b8ccbe5330ff0c6075ef773f2