Revert SwAccessibleCell::getCurrentValue to return a numeric value again

76c549eb01dcb7b5bf28a271ce00e386f3d388ba "Integrate branch of IAccessible2" had
changed it to return a string, but that neither matched the documentation of
css.accessibility.XAccessibleValue nor did it satisfy JunitTest_sw_unoapi.  This
needs further clarification apparently.

Change-Id: Ib09f7549db7ba5155798d5917908c7ce50e7a592