Export PRODUCT_COPY_FILES to Soong

The entries will be used to auto generate the prebuilt_* modules in the
filesystem_creator module to be added as deps of the autogenerated
filesystem modules.

Test: m nothing
Bug: 375053752
Change-Id: I01cfc73929d8eb7c8382b78ef9ae73c1a0fc4d3b
diff --git a/android/variable.go b/android/variable.go
index 13d5f05..34a3d30 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -591,6 +591,8 @@
 
 	ProductPackages      []string `json:",omitempty"`
 	ProductPackagesDebug []string `json:",omitempty"`
+
+	ProductCopyFiles map[string]string `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {