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
diff --git a/automotive/vehicle/2.0/default/Android.bp b/automotive/vehicle/2.0/default/Android.bp
index f9c25d1..2050038 100644
--- a/automotive/vehicle/2.0/default/Android.bp
+++ b/automotive/vehicle/2.0/default/Android.bp
@@ -62,6 +62,7 @@
         "impl/vhal_v2_0/EmulatedVehicleHal.cpp",
         "impl/vhal_v2_0/VehicleEmulator.cpp",
         "impl/vhal_v2_0/PipeComm.cpp",
+        "impl/vhal_v2_0/ProtoMessageConverter.cpp",
         "impl/vhal_v2_0/SocketComm.cpp",
         "impl/vhal_v2_0/LinearFakeValueGenerator.cpp",
         "impl/vhal_v2_0/JsonFakeValueGenerator.cpp",
@@ -98,6 +99,21 @@
     test_suites: ["general-tests"],
 }
 
+cc_test {
+    name: "android.hardware.automotive.vehicle@2.0-default-impl-unit-tests",
+    vendor: true,
+    defaults: ["vhal_v2_0_defaults"],
+    srcs: [
+        "impl/vhal_v2_0/tests/ProtoMessageConverter_test.cpp",
+    ],
+    static_libs: [
+        "android.hardware.automotive.vehicle@2.0-default-impl-lib",
+        "android.hardware.automotive.vehicle@2.0-libproto-native",
+        "libprotobuf-cpp-lite",
+    ],
+    test_suites: ["general-tests"],
+}
+
 cc_binary {
     name: "android.hardware.automotive.vehicle@2.0-service",
     defaults: ["vhal_v2_0_defaults"],