Preventing screenshotting of a web application is a challenging task.
At the browser level, there are limitations to ensure complete prevention.
One possible solution could be developing an Android Application that renders your webpage. By implementing an App Security Policy, you may restrict the ability to take screenshots within the app.
However, it is important to note that physical device photos can still capture the content.
Ultimately, there is no failsafe method to prevent all forms of screenshotting in any web application. Anything viewable by the human eye can potentially be captured in some way.
Explore Microsoft's Article on App Security Policies for more information