floss: Update socket manager implementation

Refactor socket manager interface to expose higher level apis. The major
changes in this CL:

* Expose a BluetoothServerSocket and BluetoothSocket via a set of apis
  similar to what is exposed via BluetoothAdapter and BluetoothDevice on
  Android. These are all exposed via a single SocketManager trait.

* All socket apis require a callback id and sockets are always
  associated with that callback. When the callback disconnects, any
  sockets associated with it are removed.

* RPC apis will run in the RPC runtime but socket accept/polling will
  occur on a runtime specific to the socket manager.

Bug: 233123287
Tag: #floss
Test: ./build.py and ran binary on device (feature not tested)
Change-Id: Id0a827565b87d6af097bf34eb6be3ec60f297ea3