Support adding custom tags to APIs.

Refactor the argument parsing code to support more flexible options.
Now, instead of:
  --unsupported-ignore-conflicts apis.txt

We do:
  --unsupported apis.txt --ignore-conflicts

And similarly for --packages. Flags now come in groups, starting with
one of ALL_FLAGS and continuing to the next such flag. Any of
--ignore-conflicts, --package or --tag X apply to the previous one of
ALL_FLAGS.

Also add the --tag flag used to tag APIs with a custom flag. This is
used to tag removed APIs as such.

Test: m -j out/soong/hiddenapi/hiddenapi-flags.csv
Bug: 171300342
Change-Id: I59e6c365c46282f4489d71e7acac2ae43e5907d2