ooxml: Preserve color transformations for shape theme colors

Colors can have modifiers like in the following example:

  <a:schemeClr val="accent6">
    <a:lumMod val="40000"/>
    <a:lumOff val="60000"/>
  </a:schemeClr>

In the case of RGB colors, the transformations are merged within the
RGB color itself on import, so there's no need to preserve the
original transformations, but that's necessary in the case of scheme
colors.

Slightly modified an existing unit test to check this feature too.

Change-Id: I3a03a56f2b633f283c392e54842b326bd4df316b