Floss: Fix PropMap conversion from RefArg

A PropMap is a D-Bus dictionary so it's expected that the data already
contains Variant wrapping each value in the key/value pairs. The bug was
that we tried to wrongly wrap that Variant with another Variant wrapper.

Bug: 217273154
Tag: #floss
Test: Manual - unit test and chrome vs floss test

Change-Id: I536d0c90b8b8800a0b0cae4d0ea3471ef0f6f765