test_vendor: Refactor test channel

The test channel can always be enabled when using the AsyncManager.
 - Remove enabled_ and port_
 - Allow multiple connections
 - Pass file descriptors as parameters instead of private variables
 - Add a clean up function to the Test Channel
 - Add a static class in bt_vendor.cc and always EXPORT the entry point
 - Move the vendor callbacks and the global pointer to the vendor
   manager object from vendor_manager to bt_vendor

Change-Id: I3e0dee846eb89f434893603a705c2b13219272be
Signed-off-by: Myles Watson <mylesgw@google.com>
Signed-off-by: Jorge E. Moreira <jemoreira@google.com>