Create a python script to list files given a directory as an input

The script will be used in generating a file that lists generated stub
files from metalava, which will be passed to soong_zip to extract
selected stub files when generating stubs from java_api_library modules
with incomplete api surface text files.

Test: build java_api_library modules in aosp/2487335
Change-Id: I36b830f14c714315f81e3fd8608d84c29f673f83