During the import, a cached matrix value only has an empty matrix
of correct geometry, plus the token type of the top-left cell.  The
rest of the elements are imported as hybrid values.  For now, this
seems to do the trick.

In the future we may want to change it to fully populate the matrix
cache value during the import, and skip setting the hybrid values for
the non-top-left elements.

This commit also make several other trivial changes:

* Mark pRawToken mutable so that we can mark those IsFoo() methods
  const.

* Move the ScCompiler instance from static instance to member of
  ScXMLImport. Since we don't need the instance to persist once the
  import is over, this is more appropriate.

Change-Id: I1abde03c0fcd91b02ef4dbf8b5526f7965eaf19c