Add DetachableBaseHandler to ash/

Tracks HammerdClient and PowerManagerClient to detect if a
detachable base (hammer) is connected/paired with the device.
It keeps track of the last connected base per user - this
information will be used to notify the user when the attached
base is changed.
DetachableBaseHandler does not track the active users, or session
state - it depends on its clients to determine when a base is
considered used/trusted by a user.

BUG=796300,796342

Change-Id: Iaa3e8114dfb65a6e5654b166c4bc74522b30c80e
Reviewed-on: https://chromium-review.googlesource.com/930182
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540560}
14 files changed