runtime: android x86 support

[crawshaw]

Tracking bug. There are a handful of x86 android devices in the wild, but what makes this target interesting is the Android Emulator. By far the best emulator uses Intel's HAXM kernel plugin to run android via hardware-assisted virtualization.

It also makes it possible to setup an android builder as two docker images (build host and android client) which would be much easier for other's to bring up and maintain.

[jeffallen]

In the meantime, it would be useful to note in the docs that "gomobile install" will only work if you have either real hardware, or an ARM-based AVD. You can download the arm AVD with Android Studio and use it instead of the default x86 one.