floss: Support GATT characteristic start/stop notification callbacks

Allow Floss to recognize when local Client Characteristic Configuration
Descriptors are written to, so notifications about these events can be
sent to upper browser layers.

BUG=b:323943663
TEST=passing unit tests

Change-Id: I5fb8617a12876b1bcc17bd0c5e3ff1ff8c5c46dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5412331
Auto-Submit: Sarvesh Kalwit <sarveshkalwit@google.com>
Reviewed-by: Katherine Lai <laikatherine@chromium.org>
Reviewed-by: Ying Hsu <yinghsu@chromium.org>
Commit-Queue: Sarvesh Kalwit <sarveshkalwit@google.com>
Cr-Commit-Position: refs/heads/main@{#1284517}