switch-on-connect: Do not overwrite user configured default sink/source

Currently module-switch-on-connect overwrites the default sink or source that the user has configured. This means that when the overwritten default sink or source becomes unavailable, the new default will be chosen based on priority and the default will not return to the originally configured value. This patch solves the issue by introducing new core variables for the sink or source chosen by the policy module which have higher priority than the user configured defaults.

Fixes: #870 (closed)