android: Allow custom artifacts to be passed to result_sink.Post().

I'd like to use this lib to upload Chrome OS tests to rdb as well. Some
of the tests have some artifacts that should be attached to their
results.

So this CL lets the caller of result_sink.Post() specify custom
artifacts.

Bug: 1129654
Change-Id: I35bc4009020d724e38adebb4af3cebd12a7b44be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2504579
Reviewed-by: benjamin joyce <bjoyce@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821786}