bluetooth: Wait for channel close event on Mac

Ensure the channel close event is received before tearing
down the channel and its delegate.

Bug: 330919360
Change-Id: I6e64de11b464c418a20ac911e415666d8f8b4af7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5406544


Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282048}