base-call: implement generic audio channel setup/cleanup handlers

Modems that require specific commands to setup or cleanup the audio
channel as soon as a call is ongoing can subclass these two new
methods.

The setup() method is considered part of the call start/accept
process, and so if it fails, the whole operation will fail.

The failures in the cleanup() method will be reported in the log, but
otherwise ignored, as this operation may be executed without any
user intervention (e.g. if the remote party hangs up).

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