Emacs

GNU Emacs is an extensible, customizable, free/libre text
editor — and more.

At its core GNU Emacs is an interpreter for Emacs Lisp, a
dialect of the Lisp programming language with extensions to
support text editing.

The features of GNU Emacs include:
* Content-aware editing modes, including syntax coloring, for many file types;
* Complete built-in documentation, including a tutorial for new users;
* Full Unicode support for nearly all human scripts;
* Highly customizable, using Emacs Lisp code or a graphical interface;
* A wide range of functionality beyond text editing, including a project planner, mail and news reader, debugger interface, calendar, IRC client, and more;
* A packaging system for downloading and installing extensions.

Notes regarding Android support:

* The on-screen keyboard is automatically invoked when a non read-only buffer is tapped, and the user option `touch-screen-display-keyboard' arranges for it always to be displayed.
* `modifier-bar-mode' may be enabled to display a panel of modifier keys beside the tool bar.
* Permissions are not granted by default, but must be explicitly enabled in system settings menu.
* External storage permissions may be granted by observing the instructions in the banner that is displayed on the splash screen if they are denied.
* Answers to other questions probably exist in the Android appendix to the bundled Emacs manual. Read it!