NativeHandle.java: dup() with O_CLOEXEC

Don't use Os.dup(), as it creates file handles which leak across exec()
boundaries. Instead, use fcntl(F_DUPFD_CLOEXEC);

O_CLOEXEC is essential for ensuring that file descriptors do not leak
across an exec() boundary. Setting O_CLOEXEC ensures that file
descriptors can't linger around unnecessarily in an exec()ed process
which doesn't use them, making more efficient use of resources.
Additionally, O_CLOEXEC is important in ensuring that untrusted
exec()ed code cannot take advantage of leaked file descriptors.

Test: Android compiles and boots
Bug: 120983106
Change-Id: I99a66834cc6b9bb25e1b4daf75384ec6a91ae9e2