This gives build/cordova two new command-line arguments: --ant and
--gradle, and will select the build type from those. As a fallback
for the Cordova CLI, the environment variable ANDROID_BUILD can also be
used, set to either "ant" or "gradle".
The default is currently "ant", but it is intended for this to change in
the future.
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.
