Update closeRfcommServer api to stopRfcommServer

There was an API introduced to create an RFCOMM server socket called
startRfcommServer. Its companion call to shutdown the server was
originally named closeRfcommServer. This is not consistent with the
start name, and this change updates the API to rename it to be
stopRfcommServer instead.

Tag: #feature
Test: Run the background rfcomm CtsVerifier test with an updated version
of CtsVerifier that uses the new API.
Bug: 218494317

Change-Id: I503ceac84af086cdea8a6a0cc7965f1ba5366c53