default device selection improvements

When selecting a default device, devices without active routes should be avoided.

A common case is with UCM where there is a node for (unplugged) microphone jack and a node for internal microphone. Usually the unplugged mic has higher priority than the internal mic and it gets selected as the default.

Relevant media-session logic is here: https://gitlab.freedesktop.org/pipewire/pipewire/-/blob/master/src/media-session/policy-node.c#L713