Protobuf message converter should be a public utility library

Test: Built, and flashed to Osprey (go/enable-google-vhal-on-osprey)

```
# See value changed in Vehicle HAL tab, KitchenSink app:
$ python packages/services/Car/tools/emulator/prop_event_simulator.py
--property
VEHICLEPROPERTY_HVAC_AC_ON --area 0 --value 1

# unit tests
$ atest android.hardware.automotive.vehicle@2.0-default-impl-unit-tests
```

Change-Id: I3841870614f316f2f0e2f54283674223a0b036f4