Use argparse in merge_csv.py.

- Allow passing list of explicit fieldnames, instead of sorting header names.
- Allow passing a single zip file with all CSV files to merge.

Bug: 145132366
Test: diff -w out/soong/hiddenapi/hiddenapi-greylist.csv
Change-Id: I7c6db369069452293c15eec290c635fb3879275d
1 file changed