It is tricky to prevent your adwords ads from being displayed on mobile apps (android apps). By default, your ads will be displayed on mobile apps and you need to remove the ads manually using the steps below:
1. From the Display Network tab, go to "Campaign Exclusions."
2. In the "Placements" section, paste adsenseformobileapps.com
You can also prevent your ads from being displayed on one or more mobile apps using the steps below:
In Display Network campaigns, if you want to exclude individual mobile apps rather than all mobile apps, you can use placement exclusions for mobile apps. Add these exclusions in the Display tab under "Ad group exclusions" or "Campaign exclusions".
huh, sometimes, it is better to remove the mobile app ads as all your budget is eaten by the mobile apps!!
Note: You can still opt for your ads being displayed on mobiles and tablets in addition to desktops under "Devices" tab. These ads are different from the mobile app ads.