Avoid redundant post message from WindowStateResizeItem

ClientTransactionItem is already run on main thread. If the
UI thread also uses main thread, it is unnecessary to post.

Otherwise it may cause flicker/overhead when handling
a ClientTransaction with multiple items:
 ConfigItem -> request draw
 ResizeItem -> post message (dispatchResized)
  -> The draw request from ConfigItem is executed.
     That becomes an additional frame with intermediate state.
 Resize message -> request draw (handleResized)
  -> draw again with the latest state

Bug: 298203767
Bug: 312457036
Test: Enable windowStateResizeItemFlag.
      Expand notification shade and rotate device.

Change-Id: I561335ced4e13355df233f3921b9453d3bf7196c