Aphorisms are good for that! I don't have a phone, but e.g. I've been reading The Art of Worldly Wisdom (17th C Spanish book) as a toilet book for a year or more, it's wonderful. I have a tiny book version, maybe 6x4x1cm. It's all in little sections of a half-dozen lines, great for reading 20 seconds or a couple of minutes at a time. An e-version would be great too I guess. So much wisdom in that book.

Also other aphorists - La Rochefoucauld, La Bruyere, Lichtenberg etc. La Rochefoucauld is savagely dark and funny about you (yes, you); La Bruyere similar but sunnier, fairer; Lichtenberg's kind of like an 18th C hacker writing - a scientist/mathematician writing his little thoughts about everything.

Almost all of Nietzsche's writing is in aphorisms or short paragraphs, he'd be great for that. I find him endlessly fascinating on a thousand subjects, and very funny too.