Add recent tasks to Taskbar

- Also adds a divider between Hotseat and Recents
- Dedupes Recents from Hotseat

Test: Open some recent tasks, ensure they are deduped
from Hotseat and also handle < 2 tasks.

Bug: 171917176
Change-Id: Ia782c6ccbcda94cfd844aad04dc3d25a3f072c2b