floss: Serialize Option<T> and std::fs::File in dbus

In order to represent open file descriptors, add serialization support
for Option<T> and std::fs::File (eventually, we will have
Option<std::fs::File>).

Bug: 233123489
Tag: #floss
Test: ./build.py
Change-Id: I0d23f257ae2c1b6aee10edcc1adcd3ee8840a485