Introduce unique_fd

unique_fds are wrappers for integer file descriptors that automatically
close when they go out of scope. They are movable but not copyable and
generally mimic the semantics of std::unique_ptr

Change-Id: I0657b848b6536d5ad9fc06686e240ea5c387b0ac
Test: Considerable exercise in new AIDL integration tests
Bug: None
Signed-off-by: Casey Dahlin <sadmac@google.com>