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This implements attachment handling for outgoing transfers. It reuses
the key verification step of incoming payloads and sends the
introduction payload to the remote device.

Bug: 1085067
Change-Id: I3534d60e5ad40a09d692278e312fb2d3dd3fb4aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352877
Commit-Queue: Richard Knoll <knollr@chromium.org>
Reviewed-by: Alex Chau <alexchau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800228}