Support uint index values in primvars

When we convert alembic scenes to usd through houdini, we can get indexed primvars where the indices are authored as uint values. This isn't converted properly to arnold, as opposed to when values are integers

Reproduce the conversion using a Houdini-generated Alembic scene converted to USD, focusing on indexed primvars whose indices are uint values. Trace the Arnold conversion path and compare it with integer indices; done means uint-indexed primvars convert correctly like integer-indexed primvars.

Written by the indexing model from the issue text.