Move TestHciLayer into a separate file

Previously we were copy/pasting it everywhere. Not only was this huge code
duplication, it also violated ODR since we were not always correctly namespacing
it. This CL wraps all its copies inside an anonymous namespace, and also exposes
it in bluetooth::hci in a separate file.

Test: all existing
Bug: 247839849

Change-Id: Icff2eafa5f74baae8d8f1af79395b9359ce9e19f