examples: bazel run that boots a hermetic emulator (demo delta)

[AttilaTheFun]

A small demo delta (stacked on the Android PR bazelbuild#1818) showing what it'd take to
make the example runnable with zero external setup — no Android Studio, no device:

bazel run //examples/cross_compilation/android_app:run

It installs the APK on a connected device/emulator and launches it; if nothing is
connected it boots a hermetic emulator first (downloaded emulator + AOSP
system image), so the whole Kotlin → Swift JNI → Swift chain runs from a clean
checkout. Verified end-to-end — booted, installed, launched:

The delta (4 files)

• emulator.bzl — a dev module extension that downloads the emulator + a system
  image (the two pieces @androidsdk doesn't provide; adb comes from @androidsdk).
• android.MODULE.bazel — use_extension + use_repo for those two repos.
• run.sh + a sh_binary(name = "run") — boot-if-needed, install, launch.

Caveats (why it's a separate PR, not folded into bazelbuild#1818)

• macOS/arm64 only — the emulator and image URLs are pinned for darwin_aarch64,
  to keep the delta small. A real version would select() per host OS/arch.
• Not wired into CI — it boots a GUI emulator and is for local use; the build
  is still covered by app_build_test.
• Adds a couple of large downloads (emulator + system image) on first bazel run.

Mostly meant to show the shape/cost. Happy to fold it into bazelbuild#1818, generalize it
across platforms, or leave the example build-only — your call.

[AttilaTheFun]

Folded into the base Android PR (bazelbuild#1818) — keith's fine with including the hermetic emulator example there. The commit is now part of bazelbuild#1818.