Fix the workspace build off-device

Two breaks that only appeared on a full `cargo test --workspace`.

`slint::android` exists only when compiling for Android, so darkroom-android
failed to compile on the host even though it is a workspace member. The entry
point is now gated on the target rather than on a feature.

The timeline forwarded `scrub` where the Timeline component declares
`scrub-to`, which the Slint compiler rejects.

Assisted-by: LLM

docker/android/Dockerfile

@@ -42,6 +42,8 @@ ENV DEBIAN_FRONTEND=noninteractive \
 # ---------------------------------------------------------------------------
 RUN apt-get update && apt-get install -y --no-install-recommends \
         ca-certificates curl unzip git \
+        # zip: package.sh adds the .so and dex to the aapt2-linked APK
+        zip \
         openjdk-${JDK_VERSION}-jdk-headless \
         # Slint / winit build-time needs
         pkg-config libfontconfig1-dev \