feat(android): follow the activity lifecycle and build an APK (Issue #42)

- The drawing surface is created once the native window exists (Resumed)
  and released when the app goes to the background (Suspended); before
  that everything is saved, as Android may stop the app at any time
- Android's back button acts like Escape instead of closing the app
- Panics are written to logcat; no update check on Android yet
- scripts/package-android.sh builds target/ink-notes_<version>_arm64.apk
  without Gradle (NDK, aapt2, zipalign, apksigner); signing key from
  ANDROID_KEYSTORE/ANDROID_KEYSTORE_PASS/ANDROID_KEY_ALIAS, otherwise
  the debug key

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -69,7 +69,7 @@ wayland-backend = { version = "0.3", features = ["client_system"] }
 wayland-protocols = { version = "0.31", features = ["client", "unstable"] }
 
 # Android (Issue #42): winit bound to the native activity. The library is built as
-# cdylib with `cargo rustc --lib --crate-type cdylib --target aarch64-linux-android`
+# cdylib and packed into an APK by scripts/package-android.sh
 [target.'cfg(target_os = "android")'.dependencies]
 winit = { version = "0.29", features = ["rwh_06", "android-native-activity"] }
 android_logger = { version = "0.13", default-features = false }