Well I've been researching this and I wasn't able to prevent texture reloading with interstitial ads using either admob and startapp.
Then I tried to use Leadbolt, and it's simply overlaying an ad in front of the app but it's not an activity, so I don't have to reload the textures.
If someone finds another ad network which works in a similar way (doesn't force texture reload) it would be great to know :P