Ensure not to kill process when there's non-killable activity

The process should only get killed when all activities get onStop.
Current UX ensures that since all activity will get onStop in recents
system UI. The code here will cause no difference before we envolve
other ways for user to kill a process.

For multi-tasking and multi-display, this is needed to prevent from
voilating the lifecycle related API design.

This is a starter tasks to improve large display UX, and a re-do of
fe0af5fb1be4ad253774a5d6472eed8c76bd0885.

Test: go/wm-smoke
Test: Manual. life cycle is correct when close an app on ARC++.
Bug: 111840655
Change-Id: I6311c973eba8795251d60ef6b0d44655ef836c5e