Run the download_android_tools.py in a gclient hook. (flutter#3050)

Originally, this was a step that had to be carried out manually. Presumably because we did not want developers to download the Android SDK and NDK if they were not targetting Android. In practice, this is rarely the case. Even the buildbots unconditionally run the script (though they don’t need Android artifacts on the Mac since there is no Mac to Android builder).

CONTRIBUTING.md

@@ -14,7 +14,7 @@ Things you will need
  * Python (used by many of our tools, including 'gclient').
  * curl (used by `gclient sync`).
 
-You do not need [Dart](https://www.dartlang.org/downloads/linux.html) installed, as a Dart tool chain is automatically downloaded as part of the "getting the code" step. Similarly for the Android SDK, it's downloaded by the build step below where you run `download_android_tools.py`.
+You do not need [Dart](https://www.dartlang.org/downloads/linux.html) installed, as a Dart tool chain is automatically downloaded as part of the "getting the code" step. Similarly for the Android SDK, it's downloaded by the `gclient sync` step below.
 
 Getting the code and configuring your environment
 -------------------------------------------------
@@ -46,7 +46,6 @@ target_os = ["android"]
  * `cd src/flutter` (Change to the `flutter` directory of the `src` directory that `gclient sync` created in your `engine` directory.)
  * `git remote add upstream [email protected]:flutter/engine.git` (So that you fetch from the master `flutter/engine` repository, not your clone, when running `git fetch` et al.)
  * `cd ..` (Return to the `src` directory that `gclient sync` created in your `engine` directory.)
- * Run `./tools/android/download_android_tools.py` to add Android dependencies to your tree. You will need to run this command again if you ever run `git clean -xdf`, since that deletes these dependencies. (`git clean -df` is fine since it skips these `.gitignore`d files.)
  * Add `.../engine/src/third_party/android_tools/sdk/platform-tools` to your path so that you can run the `adb` tool more easily. This is also required by the `flutter` tool, which is used to run Flutter apps.
  * Make sure you are still in the `src` directory that the `gclient sync` step created earlier.
  * If you're on Linux, run `sudo ./build/install-build-deps-android.sh`

DEPS

@@ -160,6 +160,14 @@ hooks = [
         'src/build/landmines.py',
     ],
   },
+  {
+    'name': 'download_android_tools',
+    'pattern': '.',
+    'action': [
+        'python',
+        'src/tools/android/download_android_tools.py',
+    ],
+  },
   {
     # Pull clang if needed or requested via GYP_DEFINES.
     'name': 'clang',