Always convert AllowedValues to a list

Passing e.g. a string as the AllowedValues list can appear to work, since
str.__contains__() performs a substring match. It's better to fail fast, so
convert AllowedValues to a list regardless of its initial type.

Change-Id: I74783e6a7c1743ead3f086116dd3850afeb7a028
Merge "Always convert AllowedValues to a list"