https://kodi.wiki/view/Smartphone/tablet_remotes
You might have to disable HDMI CEC if the Shield shuts down when you switch the TV OFF.
check developer setting to prevent box from shutting down, don't know about kodi in background, maybe there is something in dev options for that too
EDIT: though I think Kodi 18 Leia RC runs in bg if kodi is playing something
You will want to "Allow remote control via HTTP", give it a port, username and password. From there, load up any web browser , enter in the IP of the media player, should be able to browse everything Kodi can see as well as play it back, without the TV even on.
NOTE: For some reason, HTTP/UPnP Remote apps dont always work with Kodi for Android, but work just fine when its Kodi for Linux or Windows.
I would invest a few bucks into the Yatse app, it allows you to do a lot of things with Kodi the standard remote app can't.
As long as the Shield is on you should be able to do this. Haven't tried it tho.