I guess droidcam should depend on media-video/v4l2loopback . Because without installing media-video/v4l2loopback and `modprobe v4l2loopback` droidcam doesn't work.
droidam bundles its own version of v4l2loopback named v4l2loopback-dc. You should modprobe that module instead of the one from media-vide/v4l2loopback.
added this command to https://wiki.gentoo.org/wiki/Droidcam#Kernel