Simplify WindowState#reportResized

- System server no longer contains customized windows
  which extend IWindow.Stub directly (very old lockscreen).
  Since there is no arbitrary implementation of resized,
  it is unnecessary to post resized at the entrance because
  ViewRootImpl will dispatch the message to UI thread.
- As above, because the local async case is gone, it is safe
  to update mLastConfigReportedToClient directly so we don't
  need to create MergedConfiguration every time.

Bug: 139522754
Test: atest AppWindowTokenTests#testLandscapeSeascapeRotationByPolicy

Change-Id: Ib1ea0b07add562c40466a0a8792221768e404c62