Reproducible-build fix so the F-Droid build can be verified against this
signed APK.

- The launcher icon's inner tonal wash is now a flat fill instead of a
  gradient. A gradient needs API 24, so with minSdk 23 the build silently
  rasterised the icon into six density PNGs whose bytes varied with the
  build machine. The icon looks the same; six generated PNGs are gone.
