Use product variables from the overridden apex

override_apex's bp2build converter had a bug where it was looking at
the product variables for the override_apex module itself instead of
for the base module it is overriding.

Fixes: 271424349
Test: go test
Change-Id: If1e2653d3751fa908faf0ab97dfa2e943ebe98ec

etc/prebuilt_etc.go

@@ -717,7 +717,7 @@ func (module *PrebuiltEtc) Bp2buildHelper(ctx android.TopDownMutatorContext) *ba
 			}
 		}
 
-		for propName, productConfigProps := range android.ProductVariableProperties(ctx) {
+		for propName, productConfigProps := range android.ProductVariableProperties(ctx, ctx.Module()) {
 			for configProp, propVal := range productConfigProps {
 				if propName == "Src" {
 					props, ok := propVal.(*string)