Add android/README.md: backend channel routing, the 70 nativeBackendMethods with handler file and reply kind, host-to-Dart events, the __json_file spill, progress EventChannels (400 ms Android, 800 ms iOS), the two foreground services, the SAF model, the Rust bridge files, and the iOS routing differences.
Derived from MainActivity.kt and the MainActivity*.kt handler files, MainActivityProgressStreams.kt, SelectedCoreBackend.kt, RustCoreBackendApplication.kt, NativeDataJobs.kt, SpillJsonWriter, DownloadService.kt, BackgroundWorkService.kt, MainActivitySafScan.kt, MainActivitySafIo.kt, android/app/build.gradle.kts, proguard-rules.pro, ios/Runner/AppDelegate.swift, CoreBackend.swift and lib/services/platform_bridge*.dart.
