This headers are used by Inner Core and Inner Core native mods to link gnustl_shared.so and minecraft library, based on it. This headers can be used along with standard headers without conflict, as they are using separate namespace.
When using toolchanin, create directory toolchain/stdincludes/gnu_stl and copy stl directory from this repo into it. You can then use it like this #include <stl/vector.h>.
You can also copy stl directory to your sources root, but in this case you will need to include it as header from your sources.
IMPORTANT NOTE: you should link gnustl_shared to your native library for this to function normally!
Namespace std::__ndk1 declared in headers is identical to std namespace, so if you need to use for example vector from this namespace, you include <stl/vector.h> and use std::__ndk1::vector<int>.
But consider, that types, declared in std::__ndk1 differ in internal structure from ones in std namespace and cannot be replaced by their counterpart.
Never use using namespace std::__ndk1 to reduce amount of code, this is a bad practice even in case of one standard namespace, but in this case, when you have 2 standard namespaces, it will become a terrible mess.
Consider using global macro instead, like so:
// std::__ndk1::string -> STL::string
#define STL std::__ndk1
String conversion can be easily done via data() method:
#include <stl/string>
std::string s = "a";
STL::string stl_s = s.data();
std::string std_s = stl_s.data();