Support system/vendor for devices without a vendor partition

Test: lunch aosp_arm-eng; m -j vulkan.default
Change-Id: I91eabf98f53c455ec4ade95215f11a6f89631747
diff --git a/android/config.go b/android/config.go
index 4a59bad..8f531cc 100644
--- a/android/config.go
+++ b/android/config.go
@@ -443,6 +443,13 @@
 	return arches
 }
 
+func (c *deviceConfig) VendorPath() string {
+	if c.config.ProductVariables.VendorPath != nil {
+		return *c.config.ProductVariables.VendorPath
+	}
+	return "vendor"
+}
+
 func (c *deviceConfig) VndkVersion() string {
 	if c.config.ProductVariables.DeviceVndkVersion == nil {
 		return ""