Do not use wm lock in TaskChangeNotificationController

Because it dispatch listeners on display thread, it can reduce
lock contention of wm lock and the delay of subsequent messages.

Also simplify logWindowState:
 - Instead of post to display thread and acquire lock to get wm state
   to log, just get the simple data in wm lock and post to fg thread
   to log without lock.
 - Pinned task no long has focusable activity, so it won't be the
   top focused task. Then findRootTaskBehind can be removed.

Bug: 193661571
Bug: 204870457
Test: CtsWindowManagerDeviceTestCases
Change-Id: I41599e6b6c1ec50c6fb46151a8538c3e1b6eea67