Implement new feature flag system (re-land) (#42678)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/42678

Changelog: [internal]

This is a re-application of https://github.com/facebook/react-native/pull/42430, which had to be reverted because of crashes in optimized builds on Android:

```
Abort Reason: terminating due to uncaught exception of type facebook::jni::JniException: java.lang.ClassNotFoundException: com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsProvider
```

The root cause of that was that that class was removed because it wasn't statically referenced from Kotlin/Java, but it was dynamically referenced from C++ (in `ReactNativeFeatureFlagsProviderHolder.cpp`).

This applies the same changes + adds `DoNotStrip` annotations for the affected class and all its methods.

Reviewed By: huntie

Differential Revision: D53122992

fbshipit-source-id: efc4d5636a3f2d39b86e9c098bff408b6688b80b