Remove duplicates from VendorVars maps

Some products access the SOONG_CONFIG variables
directly, instead of using soong_config_set. When
they do that, they could end up with duplicate values
in those variables, causing duplicate keys to be
generated in a json map.

Use $(sort) to dedup the map keys before writing them
out.

Bug: 249685973
Test: Presubmits
Change-Id: If0c16377bdfbd3f836ebec9262bec7cf562f339c