Fix destruction of NumberFormat (#523)

Summary:
Pull Request resolved: #523

`NumberFormat::Impl` contains a JNI reference, and must be destroyed
in a context registered with the JVM. For now, do this using
`jni::ThreadScope::WithClassLoader`, since it avoids us needing to
add special logic to handle this in the GC.

Reviewed By: mhorowitz

Differential Revision: D28791955

fbshipit-source-id: ad81403f0a6482924721e4603f6ab8146d30dedb

lib/Platform/Intl/PlatformIntlAndroid.cpp

@@ -508,7 +508,9 @@ struct NumberFormat::Impl {
 
 NumberFormat::NumberFormat() : impl_(std::make_unique<Impl>()) {}
 
-NumberFormat::~NumberFormat() {}
+NumberFormat::~NumberFormat() {
+  jni::ThreadScope::WithClassLoader([&] { impl_.reset(); });
+}
 
 vm::CallResult<std::vector<std::u16string>> NumberFormat::supportedLocalesOf(
     vm::Runtime *runtime,