Add GetDescriptors to BluetoothLocalGattCharacteristic

This allows arc_bluetooth_bridge to remove BlueZ specific code,
which currently causes a crash in FindCCCD.

BUG=b:344663943
TEST=build and unit tests

Change-Id: Ie5355dc8be7acd12e2a55cf9504be2c548e13a90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5604942


Commit-Queue: Katherine Lai <laikatherine@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Chad Duffin <chadduffin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1311699}