Add explicit default 2015.

* Upstream config uses rustc default.

Bug: 142208376
Test: mm in rust projects
Change-Id: I0582118e7689f558ba3f5e7b3807354ac84d09b9
diff --git a/Android.bp b/Android.bp
index 5199e25..6e218b8 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1,6 +1,10 @@
 rust_library_rlib {
     name: "libunicode_xid",
+    host_supported: true,
     crate_name: "unicode_xid",
     srcs: ["src/lib.rs"],
-    host_supported: true,
+    edition: "2015",
+    features: [
+        "default",
+    ],
 }