[api-merge] Update "constant" values to mirror latest API levels (#9004)

Context: #9000
Context: ac3b405

In Issue #9000, we learned that:

  - Constant values can change between Android API levels
  - `api-merge` does not update constant values in the final `api.xml`
    if they do change

Update `api-merge` to update constant values if they change, and update
`api-VanillaIceCream.xml` with the new `api-merge` logic.

build-tools/api-merge/ApiDescription.cs

@@ -147,6 +147,9 @@ public void Merge (XDocument n, string apiLocation)
 							}
 							*/
 
+							// Yes, "constant" values can change between API levels.
+							smember.SetAttributeValue ("value", nmember.Attribute ("value")?.Value);
+
 							UpdateDeprecatedSince (smember, nmember, platform);
 
 #if KEEP_OLD_WRONG_COMPATIBILITY