Update process lru when switching focus between visible activities

Ensure that the process of the latest touched activity refreshes the
last activity time and maintains its index at the top of the LRU.

Also add a pending flag to avoid updating intermediate state, e.g.
 Launch A
 WindowContainerTransaction {
  beginDeferResume
  reorder home to top // prev=A, update home as top -> avoid this
  reorder WallpaperActivity to top // no resumed activity
  reorder A to top // prevTopActivity becomes null
  endDeferResume
 }

Bug: 421872956
Flag: EXEMPT bugfix
Test: ActivityTaskSupervisorTests#testUpdatePendingTopForTopResumed
Change-Id: Id5f0dde7124200732747185cb8998abcf1254411