examples: a runnable Android APK example (Kotlin → Swift JNI → Swift)

[AttilaTheFun]

Makes the Android example actually runnable instead of building only the JNI
.so with a prose packaging recipe. It's now a real android_binary (built with
rules_android + rules_kotlin) that loads libSwiftJNI.so and shows
"Hello from Swift, Android!" on screen — verified on an emulator:

This is the application half of the chain end-to-end:
MainActivity.kt → NativeBridge.greetingFromSwift() (Kotlin, JNI) →
libSwiftJNI.so (swift_binary(linkshared), @_cdecl) → Greeter
(swift_library).

What's here

• examples/cross_compilation/android_app/BUILD.bazel (new): android_binary +
  kt_android_library, the two cc_library .so wrappers,
  select_android_runtime_lib/transition_binary for libc++, and a
  build_test. Everything tagged manual (kept out of the //examples/...
  wildcard, like the rest of cross_compilation).
• The READMEs are rewritten to point at the runnable target.
• CI: the macos_cross_compilation task gains --config=android_example and the
  app_build_test.

The one rules_swift-specific detail

swift_binary(linkshared = True) exposes its .so via DefaultInfo, not
CcInfo, so it can't go straight into android_binary.deps. Wrapping it in a
cc_library (a prebuilt .so in srcs becomes a CcInfo dynamic library) lets
android_binary's per-ABI native split collect it into lib/arm64-v8a/.
libc++_shared.so goes in the same way via select_android_runtime_lib.

Dependency weight (the open question)

rules_android / rules_kotlin / rules_java / rules_jvm_external are all
dev_dependency = True (pinned Maven lock at //:rules_android_maven_install.json,
hermetic @androidsdk from hermetic_android_toolchains — no local SDK), so
consumers of rules_swift are unaffected. protobuf stays at 34 (no conflict).
The rules_android flags live behind a scoped --config=android_example so the
rest of the build is untouched.

Totally fine if this is too heavy to live in rules_swift — it can drop back to
the .so + recipe, or move to a standalone example repo. Opening it as a
separate commit on top of the Android PR so the delta is easy to see.

_{(Screenshot lives on the assets/android-example-screenshot branch, not in this PR's diff.)}

[AttilaTheFun]

Auto-resolved as merged: the example commit is now folded into the base Android PR (bazelbuild#1818), whose description carries the emulator screenshot. Nothing further needed here.