App Pairs: Implement Overview Actions View button

[App Pairs 6/?]

This patch includes:
- Ability to "save app pair" from Overview Actions
- 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.

Note: This patch introduces a slight regression (b/315545418) 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 DEVELOPMENT
Bug: 315545418
Bug: 315292030
Test: Manual, all action buttons still work and "Save app pair" appears and disappears correctly
Change-Id: I0eb1fa9520ba14074ede3480e771b340cf2f6e2c