Add configurable game/initial screen width and distance, with a bundled first-run config

- Video Options tab offers width and distance presets independently for
  the Initial Screen (splash/intro/menu) and Game Screen (gameplay)
  profiles, with their preset step/offset configurable via
  questshock.config.json.
- Ships res/questshock.config.default.json as the initial config on
  first run (or whenever questshock.config.json doesn't exist yet),
  refreshed from the APK on every launch without touching a user's own
  saved config.

android/app/build.gradle

@@ -104,10 +104,13 @@ def stageEngineTask = tasks.register("stageEngine", Exec) {
     inputs.dir("${projectDir}/../gl4es-src")
     inputs.dir("${projectDir}/../gl4es-patches")
     inputs.file("${repoRoot}/build-image/Dockerfile")
+    // Staged into assets/ by prepare-android-project.sh.
+    inputs.file("${repoRoot}/res/questshock.config.default.json")
     outputs.dir("${repoRoot}/build/android-engine")
     outputs.dir("${repoRoot}/build/android-gl4es")
     outputs.dir("${repoRoot}/build/android-prebuilt")
     outputs.file("${projectDir}/../engine.properties")
+    outputs.dir("${projectDir}/src/main/assets")
 }
 
 // preBuild is what every variant's compile/native-build tasks already