Java: Added ByteBufferFactory interface and sizedInputStream method. (#4379)

The ByteBufferFactory interface gives the user an option to specify
the method in which the internal ByteBuffer is allocated. This provides
flexibility in the type of ByteBuffer that can be used.
The sizedInputStream method is an alternative to sizedByteArray that
does not make a copy of the data in memory.

src/idl_gen_general.cpp

@@ -690,7 +690,7 @@ void GenStructBody(const StructDef &struct_def, std::string *code_ptr,
 std::string GenByteBufferLength(const char *bb_name) {
   std::string bb_len = bb_name;
   if (lang_.language == IDLOptions::kCSharp) bb_len += ".Length";
-  else bb_len += ".array().length";
+  else bb_len += ".capacity()";
   return bb_len;
 }