assets is a private area of an app, so, fortunately no other app can access this area of another app. I would either make a workaround via SD card (option 1) or make the tubeMaps available via the internet (not locally) and obtain images from there (option 2).
Option 1: you add a permission to access external storage. On the activity start you synchronize the images from local assets to external storage and then slightly change your code to pass another file object along with ACTION_VIEW intent pointing to external storage.
Option 2: you upload the tubemaps somewhere on the internet and provide a url to the ACTION_VIEW intent.