DO NOT MERGE - 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: Tested new activity behavior on local Ryu. Added tests for layout logic.
Bug: 32101881
Merged-In: I550f6e7ea0de3937dbf80e5f0294676cfe567d47
Change-Id: I46a537646e98b312d831510e1d331948888ae5ce