Convert 'to' value of SetTimeNodeContext based on attribute name
and move conversion code in its destructor to convertAnimationValue.
Value conversion in AnimVariantContext is also included in
convertAnimationValue and is removed together.

Change-Id: I5dc693a1bbc7df57f7506e7704f9cd4693bf2056
Reviewed-on: https://gerrit.libreoffice.org/57412
Tested-by: Jenkins
Reviewed-by: Mark Hung <marklh9@gmail.com>