b/12068020 Add a way to add uniqueness to device descriptors. Do not merge

Cherry pick from https://googleplex-android-review.git.corp.google.com/#/c/398226/

This adds an integer to the descriptor of devices without uniqely
identifying information. It will reuse values that are no longer
in use, so if you remove a single device and attach a different
identical device it will appear to be the same device.

TODO: Derive uniqueness from USB port when possible. This version
will generate different descriptors for each half of a USB keyboard
that shows up twice.

Change-Id: Ie628f19c01469f6ec2d354cd00000898ac6432fa