2-dimensional Recents activity.

This is a simple first version in the spirit of small, incremental CLs.
It is fully functional but the following will come in later changes:

* Split screen support
* Potential animations
* Alt-tab behavior
* Relayout on orientation changes

The new activity is only started when a specific system property is set.

Test: Locally on Ryu device. Added tests for layout logic.
Bug: 32101881
Change-Id: I550f6e7ea0de3937dbf80e5f0294676cfe567d47