Since the gradient frame backgrounds were added in LO 4.1, we export
this:

 fo:background-color="transparent" draw:fill="solid"

Which doesn't make a whole lot of sense as this is really a "none" fill,
and now with the backward compatibility stuff in the style import code
we get the pool default color added when setting the BackTransparent
property, and with the draw:fill="solid" it becomes visible and the
background color is now Sky Blue 1.

So try to detect draw:fill="solid" without draw:fill-color attribute
and contradicting transparent legacy attribute and nerf it.  This way
we also export draw:fill="none" again, although still with a bogus
draw:fill-color but that shouldn't cause any harm.

Change-Id: I1c2bea46ba7d9a3f042b875df0ca12c7f6037909