Allow an arbitrary tag to be associated with a bp property

This is preparation for enhancing the versioning tranformer to support
applying per property transformations. Specifically, to allow
properties to reference other libraries within the sdk.

Bug: 142940300
Test: m nothing
Change-Id: I99cdff4b407763ed395ff358d8110a63c6cf5589
diff --git a/sdk/update.go b/sdk/update.go
index 56ce1fc..0d2e4e4 100644
--- a/sdk/update.go
+++ b/sdk/update.go
@@ -314,7 +314,7 @@
 func outputPropertySet(contents *generatedContents, set *bpPropertySet) {
 	contents.Indent()
 	for _, name := range set.order {
-		value := set.properties[name]
+		value := set.getValue(name)
 
 		reflectedValue := reflect.ValueOf(value)
 		t := reflectedValue.Type()