Move the android_tools root to flutter/third_party. (#845)

I have a follow-up patch I'll need to use on `flutter/engine` moving all
references to `flutter/third_party/android_tools`.

Author: matanlurey

build/config/android/config.gni

@@ -6,7 +6,7 @@
 
 if (is_android) {
   if (!defined(default_android_sdk_root)) {
-    default_android_sdk_root = "//third_party/android_tools/sdk"
+    default_android_sdk_root = "//flutter/third_party/android_tools/sdk"
     default_android_sdk_version = "34"
     default_android_sdk_build_tools_version = "34.0.0"
   }
@@ -53,7 +53,7 @@ if (is_android) {
   android_sdk = "${android_sdk_root}/platforms/android-${android_sdk_version}"
 
   # Path to the Android NDK and SDK.
-  android_ndk_root = "//third_party/android_tools/ndk"
+  android_ndk_root = "//flutter/third_party/android_tools/ndk"
   android_ndk_include_dir = "$android_ndk_root/usr/include"
 
   android_sdk = "${android_sdk_root}/platforms/android-${android_sdk_version}"