floss: Support HashMap<K,V> in D-Bus projection property-map

Implement RefArgToRust and DBusArg traits for Rust HashMap<K, V> so that
property map can support a struct with HashMap fields, e.g.:

struct SomeData {
    services: HashMap<String, Vec<u8>>,
    manufacture: HashMap<i32, Vec<u8>>,
};

Bug: 237389694
Tag: #floss
Test: ./build.py --target test
Change-Id: Idc4eb76cf30ca56d8b039881e9424f9391ffca0e