Refactor ProductConfigProperties to use a struct key instead of an

string key with hardcoded patterns.

This fixes a bug with label list conditions_default attrs where the
attribute values get clobbered in a map with the keys
"conditions_default" (with a default empty list) and
"acme__feature__conditions_default" (with a non-empty list) when
generating the LabelListAttribute.

Test: CI
Change-Id: I5429e40f747b7a0ed559f8a468a4831cd32df2c0