fastboot: Allow fastboot to asynchronously differentiate between fastboot and fastbootd.

It's not possible to programmatically determine which fastboot mode a
device is in, without sending a getvar:is-userspace query. Unfortunately
this is not possible asynchronously, and may interrupt other queries
being processed.

This patch changes fastbootd's USB interface name to "fastbootd". Note
that tools use the protocol number/class and not this string, so it
should be safe to extend. When using "fastboot devices", the interface
name is now listed if set. Note that currently only the Linux version of
the fastboot tool is capable of reading the interface name.

Bug: 156966319
Test: fastboot devices on Linux
Change-Id: I57ccf2bec1dda573fe3ac628a646624b76f45905