Error in aapt_package when prebuilt AAR does not include any `res` files

Created by: jhansche

If a prebuild AAR file contains an empty res/ directory, the directory does not get created in the __unpack_*#aar_unzip__/res/ location. This causes the aapt tool to fail with error:

ERROR: resource directory 'buck-out/bin/__unpack_<prebuilt_aar_name>#aar_unzip__/res' does not exist

In the AndroidPrebuiltAarGraphEnhancer class, there is a step that accounts for a similar error on the assets directory:

steps.add(new MkdirStep(getAssetsDirectory()));

Adding a similar step for the res directory resolves the error.