Can Java be exploited through Android app ads?
Apps for Android are compiled into a dalvik, machine code like format before they can be run on the device.

This means that everything is compiles, and things like Java etc cannot be updated on the device - as the device doesn't actually use Java.

It could be possible for malicious code to be installed into an Android app, however Google Play does check all apps for malicious code on upload, so you should be safe as long as you download from Google Play.