Switching ANC on or off from master bud has always worked
since we could just straight up enable it and no command had to be
sent.
When switching from slave bud we had to send the command to enable
or disable it via app_anc_key. Issue is the handling of this request
is from app_anc_cmd_receive_process which is not called without
APP_ANC being defined.
This commit cleans up the way ANC is enabled and also fixes slave
bud ANC switching by implementing the actual code path :)
851fb95dcb