[a_e_k]

Alphacross [0] is the one I keep coming back to. I don't think it's open source, but otherwise it's a nice simple no-frills crossword app. No ads, minimal permissions, sips battery, downloads puzzles on-demand and then works fine offline, etc.

[0] https://play.google.com/store/apps/details?id=org.akop.cross...