Previously the Java demo app API key was hard-coded directly in a versioned string resource file, which is dangerous as it's easy to accidentally commit your key into version control.
This PR adds a new process to add your own Google Maps API key for the Java demo app:
1. [Get a Maps API key](https://developers.google.com/maps/documentation/android-sdk/get-api-key)
1. Create a file in the `ApiDemos/java/app` directory called `secure.properties` (this file should *NOT* be under version control to protect your API key)
1. Add a single line to `ApiDemos/java/app/secure.properties` that looks like `MAPS_API_KEY=YOUR_API_KEY`, where `YOUR_API_KEY` is the API key you obtained in the first step
1. Build and run
