Use py3 features in merge_csv.py.

- Use csv.QUOTE_MINIMAL strategy to only quote fields with special characters.
- Open file as 'r', not 'rb'.
- Use 'unix' dialect for '\n' as line terminator, instead of \r\n.

Bug: 145132366
Test: diff -w out/soong/hiddenapi/hiddenapi-greylist.csv
Change-Id: I888f3219beee5b83e53f05589a90a05f93399fe3