chart2: add some locking to chart::ChartType UNO service

On the libreoffice-4-3 branch, chart2_unoapi crashed with what looks
like a corrupted ChartType::m_aDataSeries triggering STL assertions.

Try to protect the mutable members with SolarMutex guards.

Change-Id: I3f2edd36b8ecf37ef60239415f70abfc8b59244d