bluetooth: Add a CCCD to the measurement interval

This change adds a CCCD (Client Characteristic Configuration Descriptor,
as defined in Vol 3, Part G, 3.3.3.3) to the measurement interval of
the fake health thermometer.  This will allow us to call
startNotifications() on the measurement interval.

BUG=719816

Change-Id: Ia7b515378be0c66e58c68d10451fbbd600e1c9c5
Reviewed-on: https://chromium-review.googlesource.com/663938


Commit-Queue: Conley Owens <cco3@chromium.org>
Reviewed-by: Vincent Scheib <scheib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501365}