Clear suggestionSpan reference once popup window is closing.

Clear suggestionSpan reference since it is not necessary once the
suggestion window is closed.

suggestionSpan is kept for identifying the target text region in
onItemClick method.

This CL is a part of groundwork for Bug 15347319 and no user visible
change is intended with this CL.

Change-Id: Ie0edf215a1b90629e280ce09df4fd6f5d95fbb06