Add a snapshot of flutter/engine/src/build to our sdk

build/android/ant/BUILD.gn

Index: build/android/ant/BUILD.gn
diff --git a/build/android/ant/BUILD.gn b/build/android/ant/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..a30fb54bf4afe369037469b1bb0fef74c17b1c05
--- /dev/null
+++ b/build/android/ant/BUILD.gn
@@ -0,0 +1,13 @@
+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+copy("keystore") {
+  sources = [
+    "chromium-debug.keystore",
+  ]
+
+  outputs = [
+    "$root_out_dir/chromium-debug.keystore",
+  ]
+}