Created by: tgummerer
Summary: When an android_binary or android_instrumentation_apk depend on libraries or android_resources which have empty res directories, aapt doesn't generate an R.txt file. Therefore the MergeAndroidResourcesStep fails with a NoSuchFileException.
Check if the R.txt file exists before trying to read it, and return an empty list if the file doesn't exist.
Test Plan: Added integration tests