adb: fix register_socket_transport related double-closes.

Multiple codepaths were closing the fd they passed into
register_socket_transport on failure, which would close the fd itself.

Switch things over to unique_fd to make it clear that we don't actually
have to close on failure.

Test: mma
Change-Id: I2d9bdcb1142c24931d970f99ebdf9a8051daf05c