Do not count closing activity as visible process

Previously, VisibleActivityProcessTracker#hasVisibleActivity can
still return true when the only activity of a process is in a
playing transition animation (since shell transition only commits
invisible until transition is done). That may lead to a timing
that background launch check still allows the process with
visible window to launch activity. For example, after pressing home
key, the closing app may still be able to launch activity at a
specific timing after APP_SWITCH_DISALLOW updates to
APP_SWITCH_FG_ONLY but the transition is not finished yet.

Bug: 396653764
Flag: com.android.window.flags.use_visible_requested_for_process_tracker
Test: WindowProcessControllerTests#testComputeProcessActivityState
Change-Id: I064ce5cbb140c68d3e1a482980df52ee5d37f361