Floss: Implement (Un)RegisterScanner

* RegisterScanner:
  Passes the request to libbluetooth's register_scanner with random app
  UUID. When libbluetooth emits on_scanner_registered we pass this event
  to IScannerCallback::on_scanner_registered.
* UnregisterScanner:
  Passes the request to libbluetooth's unregister providing the scanner
  id.
* When a callback is removed, either explicitly or implicitly due to
  client disconnecting, we clean up the registered scanners associated
  with that callback.
* Adds a basic command line to btclient for testing the implemented API.

Bug: 217273154
Tag: #floss
Test: Manual - Test with btclient

Change-Id: Ib3038979fb136cfdfe8c444cf64460d4a3177f26