Because the text editing process is baked in the base UI classes on Android (TextView), you cannot change it with an app. OEM manufacturers sometimes do because they ship different class code with their OS.
But the author works at Google, so can surely pass the message on up the chain that this design decision is stifling innovation in touch-based text editing?
I suggest focusing on the minimum viable product: an app with a single text field.
Maybe add the option to load and save txt files, but even clipboard I/O would be sufficient. Just let people play with the editor. If it's actually good, the next steps should become obvious.