Fix C64 display rendering and keyboard matrix

- Remove android:background from C64DisplayView: SurfaceView's XML
    background painted over the surface, hiding all rendered content
  - Fix pixel byte order: copyPixelsFromBuffer reads [R,G,B,A] bytes, so
    physical colors must be packed as ABGR (not ARGB) to avoid R/B swap
  - Fix six wrong C64 keyboard matrix positions (1, 2, Q, E, R, Y) taken
    from the authoritative VICE hardware matrix table

SchwertUndMagieOnPebbleCompanionApp/app/build.gradle.kts

@@ -137,6 +137,7 @@ dependencies {
     implementation(libs.androidx.core.ktx)
     implementation(libs.material)
     implementation(libs.nanohttpd)
+    implementation(libs.androidx.documentfile)
     testImplementation(libs.junit)
     androidTestImplementation(libs.androidx.espresso.core)
     androidTestImplementation(libs.androidx.junit)