Allow passing filenames to generate-NOTICE.py.

For the libandroid_support NOTICE file, we need to combine all the files
in that directory, plus the specific files pulled from bionic.

Also cleaned up some of the Python style.

Bug: N/A
Test: used for libandroid_support
Change-Id: If433e3a0f0478f06d99a9b3556e99dde06a7e5e1