Tweak default UI

It's desktop unless you're building for Android where the default it's
mobile.

You can still manually specify which UI you want and you'll get that
regardless of the platform

Only require KParts if we're building the desktop UI

Two small changes in CI:
 * Stop pretending we have a macOS CI
 * Add Android CI
    - libkexiv2, khtml and kjs would be nice but we don't have Android builds
      yet