Minor updates to fd_file.h

* Add a static IsOpenFd() method for checking if fd is open.
* File::IsOpened() now checks when underlying FD is open (not just valid)
* Centralize a definition for kInvalidFd in art projects.

Test: art_libartbase_tests
Change-Id: Ic5db8b6c80e184308c6ad8979ed31585120eb89f
8 files changed