Expose insecure rfcomm Bluetooth API.

This complements the secure rfcomm API.
The link key is unauthenticated and is subject to MITM attacks.
The link key may be encrypted depending on the type of Bluetooth device.
This helps apps which don't need the extra security or
have their own security layer built on top of the rfcomm link.

Change-Id: I71b2fa8de469ef98faa204b4dafac18a8e21d0d9