bluez5: move bluez-hardware.conf loading to the plugin

It's not really the responsibility of the session manager to load the
bluez5 device quirks, and it's easier for eg. Wireplumber if it doesn't
need to do it.

Move loading bluez-hardware.conf to be the responsibility of the bluez5
spa plugin, similarly as the alsa plugin deals with the ACP database.

Put the configuration to share/spa-0.2/bluez5, mirroring the plugin
directory structure in lib/spa-0.2/bluez5.