What about the tried and true method? The mp3. Thankfully, you can play an mp3 on an Android phone with the screen locked. Now, I couldn't just generate one mp3 manually in Audacity. I had to completely solve the problem for everyone who wants to create a timing or HIIT mp3 with any combination of starts and stops with any kind of spacing between them.
This little web page will generate a HIIT soundtrack or a Tabata Protocol soundtrack according to your specification. If you simply go to the page and click the generate button, you'll get the classic Tabata Protocol timing that Mr. Tabata himself used whilst doing his study.