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This change adds desktop-side, disabled-by-default, preliminary support
for communicating with phones over USB so that they can act as a
security key. This requires phone-side changes too that are forthcoming.

Android supports an “accessory” mode[1] that allows USB communication with
apps on the phone. This involves sending vendor commands to the device
to request that it switch into this mode and then watching the USB bus
for the device to reappear in the new configuration. Once that's
complete, data can be exchanged.

There's no good way, however, of knowing which USB devices are phones.
And it seems unwise to blast vendor commands at every attached USB
device each time we start a WebAuthn transaction. Therefore this is only
attempted for devices that are connected during the transaction. We do,
however, remember which accessory devices we previously configured and
can use them immediately if still connected.

In the future, the UI could plausibly allow the user to select the
device from a list rather than connect it. Although I'm unsure whether
that's actually useful.

[1] https://source.android.com/devices/accessories/aoa

BUG=1002262

Change-Id: Ia58632d89c83a60559b59146d69bd2638a597f59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2310193
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#795034}