[Cronet] Add UrlRequestListener.onCancelled()

Leads to a more common Java pattern where the embedder can always rely
on a callback at the end of a request (i.e. either onSucceeded, onFailed,
or onCancelled) which is a convenient place to do cleanup.
This is how Guava’s ListenableFutures/FutureCallback work.
Thanks to the change from interfaces to abstract classes, we can offer a
default implementation of onCancelled() so embedders aren't required to
always implement their own.

BUG=531538

Review URL: https://codereview.chromium.org/1386273007

Cr-Commit-Position: refs/heads/master@{#355045}

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