Which Bach piece is Saki Yoshizawa using for her ringtone?
This is the A major prelude of Book II of The Well-Tempered Clavier; I was able to identify it by inputting the first few beats of the piece into Musipedia. As noted in my question, Bach is not commonly associated with works that are titled as "etudes." However, The Well-Tempered Clavier was written originally as pedagogical material, which might warrant such a casual description.

Moreover, this might be characterized as an "etude" as Bach is often assigned to keyboard students, although the inventions and sinfonias seem much more common.