[Android] Add upstream version of studio doc.

BUG=

Review-Url: https://codereview.chromium.org/2147223005
Cr-Commit-Position: refs/heads/master@{#406050}

docs/android_studio.md

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+# Android Studio
+
+Android Studio integration works by generating .gradle files from our BUILD.gn files.
+
+[TOC]
+
+## Usage
+
+```shell
+build/android/gradle/generate_gradle.py --output-directory out-gn/Debug --target //chrome/android:chrome_public_apk
+```
+
+This creates a project at `out-gn/Debug/gradle`. To create elsewhere: `--project-dir foo`
+
+## Status (as of July 14, 2016)
+
+### What currently works
+
+ - Basic Java editing and compiling
+
+### Roadmap / what's not yet implemented ([crbug](https://bugs.chromium.org/p/chromium/issues/detail?id=620034))
+
+ - Test targets (although they *somewhat* work via `--target=//chrome/android:chrome_public_test_apk__apk`)
+ - Make gradle aware of resources and assets
+ - Make gradle aware of native code via pointing it at the location of our .so
+ - Add a mode in which gradle is responsible for generating `R.java`
+ - Add support for native code editing
+
+### What's odd about our integration
+
+ - We disable generation of `R.java`, `BuildConfig.java`, `AndroidManifest.java`
+ - Generated .java files (.srcjars) are extracted to the project directory upon project creation. They are not re-extracted unless you manually run `generate_gradle.py` again
+
+## Android Studio Tips
+
+ - Configuration instructions can be found [here](http://tools.android.com/tech-docs/configuration). Some suggestions:
+   - Launch it with more RAM: `STUDIO_VM_OPTIONS=-Xmx2048m /opt/android-studio-stable/bin/studio-launcher.sh`
+ - Setup wizard advice:
+   - Choose "Standard", it then fails (at least for me) from "SDK tools directory is missing". Oh well...
+   - Choose "Import" and select your generated project directory
+   - Choose "OK" to set up gradle wrapper
+ - If you ever need to reset it: `rm -r ~/.AndroidStudio*/`
+