pw_bluetooth_proxy: Deregister & Close channels on proxy reset/dtor

 - Rename L2capChannelManager::ReleaseChannel() to DeregisterChannel().
 - Separate L2capChannel::Close() into InternalClose() which does not
   Deregister channels and thus does not need `channels_mutex_`, and
   Close() which Deregisters then calls Close().
 - Add L2capChannelManager::DeregisterAndCloseChannels() which is called
   on ProxyHost::Reset() and ProxyHost dtor.
 - Remove H4Storage::Reset() so H4Packets can only be released by their
   destructor.
 - Friend L2capChannelManager in L2capChannel and move InternalClose()
   and Undefine() into private/protected API.

Fixed: 388088555, 381902130
Change-Id: I31cc98067da3e9d32620e772c22139eae21a771c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/259434
Reviewed-by: David Rees <drees@google.com>
Reviewed-by: Jonathon Reinhart <jrreinhart@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Docs-Not-Needed: Ali Saeed <saeedali@google.com>
Commit-Queue: Ali Saeed <saeedali@google.com>