All apps can now be dismissed via the scrim or swiping down. To properly
behave as an AFW, the apps view can no longer be within the taskbar's
layout, and will instead add and remove itself from the drag layer.
Test: Manual
Bug: 204696617
Change-Id: I86a0ffc06faa653c65b3797a57ba6512c6874221
Invoking the drawer is currently hooked up to tapping empty space on the
taskbar. Apps can be launched, and drag-n-drop split screen works. The
drawer can only be dismissed by acting on an app icon or tapping the
taskbar again.
Test: Manual
Bug: 204696617
Change-Id: I7c5fdbc7d54d8209f6f15ef80bfeb5e9b80cf647