policy-node: improve target device selection logic

-loop through all the linkable nodes, instead of using default nodes
-implement a priority scheme to pick the appropriate target linkables
-If both nodes support a passthrough format, check the intersection.
 If the intersection is empty and one of the node is passthrough only,
 skip the node as a target
-the logic now closely aligns with the media-session logic

Fixes #75
7 jobs for passthrough in 2 minutes and 17 seconds (queued for 9 seconds)
Status Name Job ID Coverage
  Container
passed container_fedora #15709953

00:00:22

passed container_ubuntu #15709954

00:00:21

 
  Build
passed build_on_fedora_no_docs #15709956

00:00:59

passed build_on_fedora_with_docs #15709955

00:01:05

passed build_on_ubuntu_no_gir #15709958

00:01:02

passed build_on_ubuntu_with_gir #15709957

00:01:05

 
  Analysis
passed shellcheck #15709959

00:00:49