Introduce ReactPackageTurboModuleManagerDelegate

Summary:
## Summary
People use `ReactPackage` instances to create NativeModules. To make the migration from NativeModule to TurboModule easy, I'm introducing a `TurboModuleManagerDelegate` that understands `ReactPackage`s, and uses them to lookup and create the Java TurboModule objects. This way, we don't have to change the way we declare NativeModules for the migration.

## TurboModule registration
Each application should have its own subclass of `ReactPackageTurboModuleManagerDelegate`. This subclass is a hybrid class with a C++ and a Java part. The Java part can (and probably should) do nothing (for now). The C++ part has to implement the `moduleName -> jni::HostObject` and `moduleName, javaInstance -> jni::HostObject` functions for all TurboModules in the application.

**Use Case: Migrating a NativeModule to TurboModule system**
1. Make the Java NativeModule extend `TurboModule`. (The reason why this doesn't happen automatically is probably because we haven't changed the Java codegen yet).
2. Modify the `moduleName -> jni::HostObject` or `moduleName, javaInstance -> jni::HostObject` functions to return the `TurboModule`.

**Use Case: Adding a new TurboModule**
1. Add the TurboModule to a `ReactPackage` in the application.
2. Modify the `moduleName -> jni::HostObject` or `moduleName, javaInstance -> jni::HostObject` functions to return the TurboModule `jsi::HostObject`.

**Note:** It's also possible to declare TurboModules by overriding the `getModule(String moduleName)` function of `ReactPackageTurboModuleManagerDelegate`. It's not a good idea, because it'll make switching between the NativeModule/TurboModule system difficult.

Reviewed By: mdvacca

Differential Revision: D15209129

fbshipit-source-id: 4b0a303595145be9b19d6f4934f956b91990f859

ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java

@@ -307,6 +307,10 @@ public class ReactInstanceManager {
     return mMemoryPressureRouter;
   }
 
+  public List<ReactPackage> getPackages() {
+    return new ArrayList<>(mPackages);
+  }
+
   private static void initializeSoLoaderIfNecessary(Context applicationContext) {
     // Call SoLoader.initialize here, this is required for apps that does not use exopackage and
     // does not use SoLoader for loading other native code except from the one used by React Native