Release built-in system properties' documentation with Javadoc

[utzcoz]

We use https://robolectric.org/configuring/ to documente built-in system properties. But some system properties are intermediate, and some are supported from a newer version. It's a big burden for us to maintain these system properties documentation to let it up to date. There is a potential solution to reduce the burden is that we add a new Java class with constants for these system properties. We update this Java class when there is a change of supported system properties, and add extra Javadoc for these constants. When there is a new Robolectric released, we can release the related Javadoc contains these system properties' documentation, and we can point out it at robolectric.org. Develoeprs/users can check related Javadoc to learn which system properties are supported in the Robolectric release that they are using.