Introduce initial A-Z app drawer to taskbar.

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