Allow mach to build FirefoxReality APK, install it and run it on oculus devices

[jdm]

It should be possible to set up a configuration setting pointing at a FirefoxReality working directory, run ./mach package --reality to invoke gradle, then run ./mach install --reality and ./mach run --reality http://google.com to control the attached device. This would be an easier workflow to manage when making changes to Servo that need to be reflected in the final FxR build that gets run on the device.

[highfive]

cc @wafflespeanut

[paulrouget]

I've documented how to build and run FxR without Android Studio here: https://github.com/servo/servo/wiki/FirefoxReality-build#installing-and-running-a-local-firefox-reality-build