some cleanup in `chrome.bluetooth`

- export from `chrome.dart`
- added top-level `bluetooth` namespace getter and make it a singleton.
- removed `Socket` class which conflicted with the type of the same name
in `socket.dart`; AFAICT the `disconnect`, `read`, and `write' functions
only need an integer `socketId`, so I updated those functions
accordingly to eliminate this conflict.  TODO: this really needs tests.