bluez.lua: set the loopback input stream media class to internal
Similar to the BT set node, this MR sets the media class of the loopback input node to "Stream/Input/Audio/Internal"
. This has the advantage to hide the loopback stream node from applications, making desktop environments to not show the 'recording from microphone' icon (Eg gnome-panel) when the bluetooth device is not recording yet. The linking policy still works because the "Stream/Input/Audio"
sub-string is still present in the new media.