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

bp2build/testing.go

@@ -454,7 +454,7 @@ func (m *customModule) ConvertWithBp2build(ctx android.TopDownMutatorContext) {
 			}
 		}
 	}
-	productVariableProps := android.ProductVariableProperties(ctx)
+	productVariableProps := android.ProductVariableProperties(ctx, ctx.Module())
 	if props, ok := productVariableProps["String_literal_prop"]; ok {
 		for c, p := range props {
 			if val, ok := p.(*string); ok {