ClearHighlight() used GetHighlightActor() to retrieve the currently
displayed highlight frame in order to determine which actor to remove
from the scene graph. However, it is perfectly legal to
SetHighlightActor() while a highlight frame is displayed (takes effect
on the next GrabHighlight()), so the currently visible highlight frame
handle needs to be saved separately in order to be properly cleared.
Change-Id: Ibf41574bac66f9375b72b31d061195d0235621aa