Strip empty spans after style attributes have been removed

We already do this for removeHtmlAttributes mode.

Add tests in ElementLinearData and ce.Surface. Get rid of unused
clipboardHtml param in ce.Surface tests as it is unused and replace
with clipboardKey flag which tests the VE->VE paste path.

Change-Id: I0cd82a37d6e765ecab489dc22f642ebbcfbb5387
(cherry picked from commit dc5269f249bafd01932cd05e6f1674b297bffe23)