floss: Support Vec<T> in D-Bus projection property-map

This adds a converter from D-Bus RefArg to Rust Vec<T> so that property
map can contain vector data, e.g.:

struct SomeSettings {
  setting_a: i32,
  setting_b: Vec<i32>,
  setting_c: Vec<OtherSettings>,
  setting_d: Vec<Vec<i32>>,
};

Bug: 193685325
Tag: #floss
Test: Build floss on Linux and AOSP

Change-Id: I516888a55e44c8e2c6322fe3fcec81f7bda9e748