Capabilities of the Android Java Class

The documentation says it's a wrapper is all.
What I'm interested in is, What are the capabilities of this class in terms of expanding the engine?
The reason I'm writing is, lately I was asking about swapping some features in Unity (e.g. the collision detection for no apparent reason, or the whole messaging and basically just use Unity as a renderer).
If my target platform is Android, can I do that by basically implementing a system using Java and just import the class into Unity? would that work?

If not, what are the capabilities of expanding Unity (from Unity personal)? I know about the plugins, but I'm unaware if they can do that.
Thanks for the info.