hidl_package_root: don't expose Full_root_path

It's an internal detail.

Bug: N/A
Test: hidl_test
Change-Id: I12b97101d56c15e1a6fbbbc2aa6f31db52776bb6
diff --git a/build/hidl_interface.go b/build/hidl_interface.go
index 3d78223..fdf22f6 100644
--- a/build/hidl_interface.go
+++ b/build/hidl_interface.go
@@ -465,7 +465,7 @@
 			panic("internal error, multiple dependencies found but only one added")
 		}
 		visited = true
-		h.properties.Full_root_option = dep.(*hidlPackageRoot).properties.Full_root_option
+		h.properties.Full_root_option = dep.(*hidlPackageRoot).getFullPackageRoot()
 	})
 	if !visited {
 		panic("internal error, no dependencies found but dependency added")