Maintain header order in merge_csv

Previously, if the --header property was not specified then merge_csv
would use a header constructed by sorting all the fields in the input
files. That required that any use of merge_csv which did not already
have headers in the required order would have to explicitly specify the
headers. That made it harder to use merge_csv as a generic tool as each
invocation needed to be aware of what headers were exported in the
output.

This change causes merge_csv to simply use the headers in the order in
which they are encountered in the input files. That removes the need to
specify the --header option when generating the index files.

Bug: 179354495
Test: m out/soong/hiddenapi/hiddenapi-index.csv out/soong/hiddenapi/hiddenapi-unsupported.csv
      - make sure that they are not changed by this change.
Change-Id: I420b7d07aea85af6372cd7580a8be5e2cc82a513