Marketing Channels can't be exposed in eVars because they're set when AA processes its collected data.
But Marketing Channels are applicable to both web and app, since they're actually applied at the report suite's level. So you should still be able to make use of Marketing Channels with your app's data, though it won't be as straightforward as with web data.
E.g. to set the app's Marketing Channel from your paid campaigns: assuming the user opens www.website.com?cid=abc123 on his phone. The website loads, and then there is code in there that automatically triggers his app to open. By right, your app's developer should be able to detect that original www.website.com?cid=abc123 URL.
It might be detected as the Referrer, in which case it should be straightforward for Marketing Channel Processing Rules to detect it and set the Marketing Channel appropriately.
If it's not detected as a Referrer, then your app's developer might need to extract that "abc123" tracking code and set it with the s.campaign variable, so that you can use it as the Tracking Code in your Marketing Channel Processing Rule.