Expose Android sensors to via Mojo services

This CL adds a sensor_service to sky/services and wires it into SkyShell
The plan is to eventually use this data to implement shake-to-refresh.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/962043002

examples/shell/sensor.sky

+<script>
+import '/sky/framework/shell.dart' as shell;
+import 'dart:sky';
+import 'package:sky/services/sensors/sensors.mojom.dart';
+
+class MyListener extends SensorListener {
+  int count = 0;
+
+  void onAccuracyChanged(int accuracy) {
+    print("onAccuracyChanged $accuracy");
+  }
+
+  void onSensorChanged(SensorData data) {
+    double value = data.values[0] + data.values[1] + data.values[2];
+    if (value > 40.0) {
+      document.querySelector('div').textContent =
+          "Shake count " + (count++).toString();
+    }
+  }
+
+  MyListener.unbound() {
+    stub = new SensorListenerStub.unbound()
+            ..delegate = this;
+  }
+
+  SensorListenerStub stub;
+}
+
+void main() {
+  var sensorService = new SensorServiceProxy.unbound();
+  shell.requestService(sensorService);
+
+  var listener = new MyListener.unbound();
+  sensorService.ptr.addListener(SensorType_ACCELEROMETER, listener.stub);
+  listener.stub.listen();
+}
+</script>
+<div>Shake me.</div>