the main purpose of this directory is 
to define a different ic_launcher icon for the build-type 'debug'.

the build-process seems to prefer files from /src/debug/res
over the ones from the /res, not totally sure, however, it
works :)

btw the dev-icon is not available as vector graphic,
the original source is in xxhdpi - 
all other resolutions are just scaled with gimp.

via http://onebigfunction.com/android/2016/10/06/flavor-flav-android-product-flavors-explained/