DO NOT MERGE - Delete the separate grid recents activity and related code

This was the first attempt at implementing grid-based Recents.
The new implementation doesn't use a separate activity and reuses a lot
more code.

Bug: 32101881
Test: Checked Recents behavior on local sw600dp device
Change-Id: Ic3535fc49f3f5448d6002d354678b633f122f57d