Ultrasound: Fix off-screen display window frame size overwrite

In certain scenarios, the frame size of a window which resides in
the off-screen display could have been overwritten by the frame
size of a window in the main display.

This fix uses a fresh Rect object for the queried frame size to
make sure it does not get overwritten.

Change-id: I06ad85fd197f41de0adecaa25e306ec6bf6e6290