Check PBAP Server before disconnecting

Update disconnect logic to ensure that the proxy to the PBAP server
exists before issueing disconnect.  This brings the code in line with
several of the connect calls.

Bug: 123700890
Test: Disable PBAP server from config, and disconnect a paired device.
Change-Id: Ie14feb5c0fb70f1ed68ba21bc12b4d3349933cc6
1 file changed