Custom environment/arguments to Android Emulator

I have IntelliJ IDEA with the Flutter plugin working on Arch Linux. By default, the plugin *cannot launch the Android emulator on Arch Linux* (hopefully that helps other users who are searching). I have two issues:

* I need to specify an older version of `libstdc++.so` to get the Android emulator to run
* If I play any music whilst the emulator is open, it goes all garbled

I have solved these by:

* Editing my `/usr/share/applications/idea.desktop`:

```
# Exec=idea.sh %f
Exec=env LD_PRELOAD=/usr/lib/libstdc++.so.6 idea.sh %f
```

* Not playing music during development 😢

It would be great if we could specify environment variables and additional arguments that the [`AndroidEmulator`](https://github.com/flutter/flutter-intellij/blob/master/src/io/flutter/android/AndroidEmulator.java) can use. With those changes I can add `LD_PRELOAD=/usr/lib/libstdc++.so.6` to the environment and `-no-audio` to the command line.

I would be happy to work on this (if it's not already configurable; Flutter n00b) but would need some guidance on how to expose UI settings boxes and how to grab them in the `AndroidEmulator` class.