Support common methods of caption window decoration in wrapper

Avoid crashing when using caption window decoration.

Bug: 409648813
Flag: com.android.window.flags.enable_window_decoration_refactor
Test: Enable non-desktop freeform mode.
      Drag to resize a freeform task.
      No exception from onDragPositioningStart
       -> mWindowDecoration.getDisplay()
Change-Id: I2e1a9a99b85589ebd8008588ba5cdb15ca5d7f2e