Add unit tests for BaseBwuHandler and BwuManager (BWU initiation only). Add fake classes and helper methods needed for testing. Refine comments and logs.
PiperOrigin-RevId: 442021842
* Fix bandwidth upgrade service ID usage. For instance, no longer use the inaccurate service ID from ClientProxy.
* Add more fine-grained upgraded endpoint tracking using both service IDs and endpoint IDs
* Only "revert" bandwidth-upgrade mediums--for example, stop accepting connections on a server socket--for a service when the last endpoint for that service is finished.
These changes are necessary to support multiple services (e.g., Nearby Share and Phone Hub) and multiple mediums (e.g., WebRTC and WLAN).
PiperOrigin-RevId: 442021257
Guogang Li