When a user clicks an interstitial ad, the browser will open or whatever. When he navigates back to my app, the interstitial ad is still open and he has to close it.
How can I prevent an interstitial ad from still being displayed after the user clicked it and navigates back to my app?
Add the following to your AndroidManifest.xml:
<activity
android:name="com.google.android.gms.ads.AdActivity"
android:noHistory="true"/>
What noHistory does:
Whether or not the activity should be removed from the activity stack and finished (its finish() method called) when the user navigates away from it and it's no longer visible on screen — "true" if it should be finished, and "false" if not. The default value is "false".
A value of "true" means that the activity will not leave a historical trace. It will not remain in the activity stack for the task, so the user will not be able to return to it. In this case, onActivityResult() is never called if you start another activity for a result from this activity.
Source: https://developer.android.com/guide/topics/manifest/activity-element.html#nohist