Implements the "Save App Pair" button in Overview Actions

Second attempt at enabling this feature. See ag/25143701, linked bugs, and revert history for details. This patch is tested with mokey devices and does not crash!

This patch includes:
- Ability to "save app pair" from Overview Actions.
- Internally the actions bar is not longer hidden for GroupedTasks; instead the individual buttons for "Select", "Screenshot", and "Split screen" are toggled off and "Save app pair" is toggled on.
- "Save app pair" option does not appear in icon menu when it is visible in the Actions Bar.
- Addition to go/quickstep/res/layout/overview_actions_container.xml to support addition of the new button.

Note: This patch introduces a slight regression where users are now able to long-press to select text on task 1 of a paired GroupedTaskView (previously we did not let them select text on either task for consistency.) Will follow up with a fix -- see bug for details.

Flag: ACONFIG com.android.wm.shell.enable_app_pairs TRUNKFOOD
Fixes: 315545418
Fixes: 315292030
Test: Manual, all action buttons still work and "Save app pair" appears and disappears correctly
Change-Id: I4b4ac525cfd5eb9cfcd757d0bffc7215b04f9a3d