Fix update process for DisplayName and Description properties

[trickpatty]

When updating DisplayName or Description (the only two localizedTraitTable-backed properties), changes were not persisting. It looks like there was a bug in the UpdateConstantValue method which caused the original value to be written back onto the property instead of the new value.

This change fixes the behavior to act as intended (and as described in the comments which were already on the method).

This change also provides a test which covers this scenario and checks that these two properties update correctly when set through the SDK.