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/android/sdk.go b/android/sdk.go
index 9e6ad16..006b3a0 100644
--- a/android/sdk.go
+++ b/android/sdk.go
@@ -182,6 +182,8 @@
 	AddPrebuiltModule(member SdkMember, moduleType string) BpModule
 }
 
+type BpPropertyTag interface{}
+
 // A set of properties for use in a .bp file.
 type BpPropertySet interface {
 	// Add a property, the value can be one of the following types:
@@ -190,9 +192,12 @@
 	// * bool
 	// * BpPropertySet
 	//
-	// It is an error is multiples properties with the same name are added.
+	// It is an error if multiple properties with the same name are added.
 	AddProperty(name string, value interface{})
 
+	// Add a property with an associated tag
+	AddPropertyWithTag(name string, value interface{}, tag BpPropertyTag)
+
 	// Add a property set with the specified name and return so that additional
 	// properties can be added.
 	AddPropertySet(name string) BpPropertySet