core: change configured_default_sink/source type to string

core: change configured_default_sink/source type to string

This allows us to restore the default device properly when a
hotpluggable device (e.g. a USB sound card) is set as the default, but
unplugged temporarily. Previously we would forget that the unplugged
device was ever set as the default, because we had to set
configured_default_sink to NULL to avoid having a stale pa_sink pointer,
and also because module-default-device-restore couldn't resolve the name
of a currently-unplugged device to a pa_sink pointer.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=89934