alsa-ucm: Make modifiers track conflicting/supported devices as idxsets

Devices already track their supported/conflicting devices as idxsets, do the same for modifiers. Like !588 (closed), this always creates the idxsets. This is intended to make something like pa_idxset_issubset(mod->supported_devices, selected_devices) (from !587 (closed)) possible, to easily choose which modifiers we should add to a profile when creating multiple profiles with conflicting devices.