Fix some android configuration

build/config/android/config.gni

 # Copyright 2014 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.
 
 # This file contains common system config stuff for the Android build.
 
 if (is_android) {
   has_chrome_android_internal =
       exec_script("//build/dir_exists.py",
                   [ rebase_path("//clank", root_build_dir) ],
                   "string") == "True"
 
   if (has_chrome_android_internal) {
     import("//clank/config.gni")
-  } else {
+  }
+
+  if (!defined(default_android_sdk_root)) {
     default_android_sdk_root = "//third_party/android_tools/sdk"
     default_android_sdk_version = "21"
     default_android_sdk_build_tools_version = "21.0.1"
   }
 
   declare_args() {
     android_sdk_root = default_android_sdk_root
     android_sdk_version = default_android_sdk_version
     android_sdk_build_tools_version = default_android_sdk_build_tools_version
 
@@ skipping 123 matching lines @@
   } else if (cpu_arch == "mipsel") {
     android_app_abi = "mips"
   } else {
     assert(false, "Unknown Android ABI: " + cpu_arch)
   }
 } else {
   if (!defined(is_android_webview_build)) {
     is_android_webview_build = false
   }
 }