This app was mentioned in 24 comments, with an average of 1.38 upvotes
Simple Notepad (https://play.google.com/store/apps/details?id=org.mightyfrog.android.simplenotepad&hl=en_US) ticks most of your requirements, including password protection. It's not open source though. Only requires SD Card permission for export/import.
Android Phones; Install Simple Notepad, it has a widget that places 'sticky notes' on your desktop. These are resizable, customisable, and can include checkboxes to mark things off, like a shopping list.
https://play.google.com/store/apps/details?id=org.mightyfrog.android.simplenotepad
It's a bit rough UI and usability wise, but quite plentiful. Widget isn't customizeable once created.
I believe there may be better note-taking widgets, but it is good with no ads.
Simple Notepad is an awesome app to take notes, to-do lists and reminders. It automatically saves them on .txt format, so you can easily transfer them to other devices without compatibilities issues. You can organize them using unlimited folders, set priorities on to-dos, and is very customizable. And is 621 kb on the Play Store (about 2 Mb installed).
Linkme: Simple Notepad
Edit: Here's the link, because the bot messed up: https://play.google.com/store/apps/details?id=org.mightyfrog.android.simplenotepad
You should try Simple Notepad, report no trackers by exodus
Try Simple Notepad . Basically the easiest notes app I've found for android. It can export and import plain text format (.txt) and is about as straightforward to use as the windows notepad app.
To answer my own question, it turns out Simple Notepad allows me to export content as a .txt file if I paste the content from clipboard.
It also has an addon for syncing with a Dropbox folder.
SIMPLENOTEPAD CAN DO IT
Just config.....SETTING→ auto-title=TIMESTAMP
https://play.google.com/store/apps/details?id=org.mightyfrog.android.simplenotepad
Really sorry that didn't turned out suitable for you. How about this?
https://play.google.com/store/apps/details?id=org.mightyfrog.android.simplenotepad
Samsung notes or Simple Notepad are 2 i know
I use Simple Notepad