floss: Use into_raw_fd to convert unixstream

Using as_raw_fd will drop the unixstream once it's out of scope.
Instead, convert it back to a standard unixstream (from tokio
unixstream) and then convert to raw fd.

Bug: 259485364
Tag: #floss
Test: Manually
Change-Id: I3a50531f3d12cc8947f35a847094693c0b7be48f