Add sample project for showcasing different R8 scenarios

[AjeshRPai]

This pull request introduces a comprehensive Android sample project (UsedByNativeSample) that demonstrates the use of the androidx.annotation.keep Gradle plugin and best practices for keeping code reachable when using reflection, JNI, and serialization with R8/Proguard. It includes a fully configured app module, native C++ integration, multiple Kotlin and Java examples illustrating reflection and code retention scenarios, and detailed documentation for setup and testing.

The most important changes are:

Project Setup and Documentation

• Added a new sample project (UsedByNativeSample) with a detailed README.md explaining how to use the androidx.annotation.keep plugin, including instructions for both Android and Java/Kotlin modules.
• Added .gitignore files for both the project root and app module to exclude build artifacts and IDE files. [1] [2]

App Module Configuration

• Created a complete build.gradle.kts for the app, enabling Compose, native builds, Kotlin Parcelize, and integrating the annotation-keep plugin.
• Added a standard AndroidManifest.xml and instrumented test (ExampleInstrumentedTest.kt). [1] [2]

JNI and Native Code Integration

• Added C++ JNI code (native-lib.cpp and CMakeLists.txt) demonstrating both downcalls (Kotlin→C++) and upcalls (C++→Kotlin), with comments on required keep rules. [1] [2]
• Implemented Kotlin classes (JniBridge, NativeLib, etc.) to interact with the native library and show JNI usage in the app UI.

Reflection and Code Retention Examples

• Added multiple Kotlin examples showing common reflection scenarios that require keep rules, such as accessing methods and fields via reflection, loading classes, and using annotations to drive reflection logic. [1] [2] [3] [4] [5]
• Included a Java annotation (UsedByNative) to mark members used by native code for retention by R8/Proguard.

Serialization and Parcelable

• Added examples for custom Java serialization (SerializableExample.kt) and Kotlin Parcelable (ParcelableExample.kt), demonstrating the need for keep rules to preserve serialization methods. [1] [2]

These changes together provide a robust reference for handling code shrinking and reflection in modern Android development.

References: [1] [2] [3] [4] [5]