core: fix duplicate values in NM-specific PropertiesChanged signals

core: fix duplicate values in NM-specific PropertiesChanged signals

GVariantBuilder doesn't care if you give it multiple dict entries with
the same key.  But that causes duplicate dict entries in the legacy
D-Bus PropertiesChanged signals that NM emits.  So instead go back to
a hash table to ensure that any previous value is dropped in favor of
the new one.