If I’m understanding your questions correctly, yes, you can create custom instrumentation that can be viewed in Insights in real-time by creating custom dashboards with your specific NRQL queries. Here is some documentation to get you started:
As for your second question, you only need to initialize the New Relic agent once, in the base activity in the onCreate method, and that will monitor any activities that are called during runtime. It is not necessary to instrument the agent in each activity of an application. Below, I’ve included some NR documentation outlining installation for Android and additional information about the SDK API.
Hopefully this helpful! Please let us know if you have any other questions.