Enable Gecko to enumerate monitor devices last

[na-g]

This is in reference to https://bugzilla.mozilla.org/show_bug.cgi?id=1624769. In Gecko we would like to list the monitor devices last (and perhaps loop-back devices?). We are not opinionated as to by what mechanism that happens: either via flags which we can use while constructing the gUM prompt, presorting them in cubeb, etc.