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Previously, the cancel implementation invoked the transfer-update
callback before cancelling any ongoing payloads via Nearby Connections.
The transfer-update callback subsequently cleaned up the payloads and
disconnected before the payloads could be cancelled; so, the payloads
would never be cancelled via Nearby Connections and the payload tracker
would never receive a cancellation notification. This is particularly
harmful to metrics.

In this CL, we
  * cancel payloads via Nearby Connections before doing anytyhing else;
  * ensure that we provide enough time for the remote device to process
    cancellation before disconnecting;
  * make some memory-management improvements, such as copying the
    ShareTarget in DoCancel()--the notification manager owns the
    reference passed into Cancel(), and that could easily be invalidated
    during the cancellation process;
  * ensure that the payload tracker does not process trivial data sent
    during a cancellation, notably a 0 value for the number of bytes
    transferred.

Manually verified all types of cancellation. Also, verified that
existing metrics and the metrics of https:/crrev.com/c/2571443 are
as expected. We still see crbug/1155412 and b/175064390, but those
are known, unrelated issues.

Fixed: 1156232
Change-Id: I0d480b25d3565a21138d1ae32377cc4f4fcd87c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577767
Auto-Submit: Josh Nohle <nohle@chromium.org>
Commit-Queue: James Vecore <vecore@google.com>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#834597}