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
Merge "Strip empty spans after style attributes have been removed"