Android: specify the supported version of Android Studio.

Now that 2.3 has been released, it will be the default version downloaded
by developers. //build/android/gradle/generate_gradle.py doesn't work
with 2.3 yet, and this should be reflected in our docs (we only support
2.2 currently).

BUG=700438

Review-Url: https://codereview.chromium.org/2758053002
Cr-Commit-Position: refs/heads/master@{#458060}

docs/android_studio.md

@@ -4,6 +4,8 @@
 
 ## Usage
 
+**Note: currently only Android Studio 2.2 is supported**
+
 Make sure you have followed
 [android build instructions](android_build_instructions.md) already.