[Code health] Convert ErrorCallback to OnceCallback.

This was recently marked as RepeatingCallback, but should actually be
OnceCallback, as it's fired at most once.

MTPDeviceDelegateImplLinux does at times try to pass this callback off
to more than one owner, however this is an implementation detail that
should not make it into the public API. To circumvent this hurdle, a
wrapper is introduced in MTPDeviceDelegateImplLinux.

Bug: 1152278
Change-Id: Ie00927fad8f8fa95961380fcd391e6b463263a23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2598010


Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840231}