Add `JByteBuffer::rewind()`

Summary:
Adds a `rewind()` method to `JByteBuffer`, which maps to `java.nio.Buffer#rewind()`.
This is useful if a `ByteBuffer` is reused for calls between Java and C++.

Differential Revision: D7435171

fbshipit-source-id: 488131d6ad2d5abb1b86a5efabc2f39ba0ab16cd

ReactAndroid/src/main/jni/first-party/fb/include/fb/fbjni/ByteBuffer.h

@@ -15,6 +15,13 @@
 namespace facebook {
 namespace jni {
 
+class JBuffer : public JavaClass<JBuffer> {
+public:
+  static constexpr const char* kJavaDescriptor = "Ljava/nio/Buffer;";
+
+  void rewind() const;
+};
+
 // JNI's NIO support has some awkward preconditions and error reporting. This
 // class provides much more user-friendly access.
 class FBEXPORT JByteBuffer : public JavaClass<JByteBuffer> {

ReactAndroid/src/main/jni/first-party/fb/jni/ByteBuffer.cpp

@@ -22,6 +22,11 @@ local_ref<JByteBuffer> createEmpty() {
 }
 }
 
+void JBuffer::rewind() const {
+  static auto meth = javaClassStatic()->getMethod<alias_ref<JBuffer>()>("rewind");
+  meth(self());
+}
+
 local_ref<JByteBuffer> JByteBuffer::wrapBytes(uint8_t* data, size_t size) {
   // env->NewDirectByteBuffer requires that size is positive. Android's
   // dalvik returns an invalid result and Android's art aborts if size == 0.