Your Mac computer (Macbook Pro, Macbook Air, or iMac) uses a variety of plists (Property List files) to store its settings.
com.google.keystone.agent.plist is used by Google’s Keystone agent, which is responsible for synchronizing contacts, calendar events, and email between your macOS computer and your Google account.
To answer your question:
No, com.google.keystone.agent.plist is not a malware. It is an agent that helps manage Google accounts on macOS.
If you’re not sure whether or not an app on your Mac is malware, you can use a malware scanner to check it for you. Malwarebytes and Sophos are both good options.