It has a stable API for Java clients on Android, if Google actually would care they could expose that one.
And in any case, creating a 2D abstraction layer stable API on top of platforms specific version APIs, specially given that all NDK APIs are C based even if written in C++, it is not rocket science, rather a matter of wanting to do it.
And in any case, creating a 2D abstraction layer stable API on top of platforms specific version APIs, specially given that all NDK APIs are C based even if written in C++, it is not rocket science, rather a matter of wanting to do it.