[build] Add stub for third_party/android_tools for updated zlib.

This copies third_party/android_ndk/BUILD.gn to third_party/android_tools/BUILD.gn. Old version of zlib used to reference former, updated version - latter.

Follow-up to https://dart.googlesource.com/sdk/+/44d977b897e780e20383c37b85e7f284239c6803.

Change-Id: I2c2537d1db572788d83ccd09fa3b1c916202cbfb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/183060
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

build/secondary/third_party/android_ndk/BUILD.gn

@@ -0,0 +1,28 @@
+# Copyright 2018 The Dart project Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/android/config.gni")
+
+config("cpu_features_include") {
+  include_dirs = [ "$android_ndk_root/sources/android/cpufeatures" ]
+}
+
+config("cpu_features_warnings") {
+  if (is_clang) {
+    # cpu-features.c has few unused functions on x86 b/26403333
+    cflags = [ "-Wno-unused-function" ]
+  }
+}
+
+source_set("cpu_features") {
+  sources = [ "$android_ndk_root/sources/android/cpufeatures/cpu-features.c" ]
+  public_configs = [ ":cpu_features_include" ]
+  configs -= [ "//build/config/compiler:chromium_code" ]
+  configs += [
+    "//build/config/compiler:no_chromium_code",
+
+    # Must be after no_chromium_code for warning flags to be ordered correctly.
+    ":cpu_features_warnings",
+  ]
+}