libcutils: ashmem check fd validity

- collect st_rdev on open, or when required, for /dev/ashmem.
- check in all cases if fd is a character device and matches rdev.
- requires that we have open, read and write access to /dev/ashmem
  to check, assumption being that if we can not, then we have no
  right playing with the associated misdirected file descriptor.

NB: ashmem libcutil library entry points can no longer be called in
    signal context.

Bug: 26871259
Change-Id: I9a17e33317a9be795131473a51c16f761b5f7407