DRTVWR-575: Use llsdutil.h functions for LLFloaterLandHoldings LLSD

LLFloaterLandHoldings::postBuild() was constructing an LLSD structure by
assigning each map entry and array element one at a time. Chorazinallen
identified a crash bug possibly caused by destroying that LLSD structure.
Diagnostically try building it using nested llsd::map() and llsd::array()
calls instead to see if that improves matters.