Uses heuristics:
- Adds javac to PATH based on default install paths on Windows
- Adds javac to PATH based on JAVA_HOME
- Adds android and adb to PATH based on ANDROID_HOME
- Sets ANDROID_HOME based on location of "android"
This eliminates the use of shelljs.exec's sync mode, which is the source
of the filehandle leaks that can cause EMFILE on OSX, and are
CPU-intensive everywhere.
Tested locally, needs poking before it gets released.
