Google has removed a feature of the Android operating system that has been used in the past in ransomware attacks.
Starting with Android O (8.0), set to be released in the fall of 2017, Google plans to deprecate the following window types: TYPE_SYSTEM_ALERT, TYPE_SYSTEM_ERROR, and TYPE_SYSTEM_OVERLAY.
These are special "system" windows that are shown above any app on the user's screen. As you'd imagine, this is highly valued realty for ransomware developers, who often aim to obtain permissions to show content via these windows.
Once they manage to obtain such permission, they use these windows to block the user's access to the rest of his phone and show ransom notes.