I am a Java Android developer.
I would really like to apply my knowledge in blockchain development, but I absolutely do not know where to start. Once upon a time, I was advised to try my hand at developing on Solidity, but I never found the time to do it.
You should first figure out what exactly you'd like to do / develop. You can build stuff based on Bitcoin, like automated faucets, instant exchangers or a Bitcoin <> Lightning bridge. For that, you'd want to learn how the
Bitcoin Core RPC interface works and read the
Core Lightning developers guide.
You could also build Core Lightning plugins to make something solely built on Lightning, which I find more exciting.
If you want to actually write code in a blockchain scripting language (for whatever reason), you could go for something like Solidity, indeed, but that's going to be altcoin-based and that market is pretty much fully saturated; in general, you should really have an idea what you want to make, first, since that dictates what tools and languages you need to learn.