Fix Java lint warnings in google maps plugin (#745)

Author: amirh

packages/google_maps_flutter/android/src/main/java/io/flutter/plugins/googlemaps/Convert.java

@@ -37,8 +37,9 @@ private static BitmapDescriptor toBitmapDescriptor(Object o) {
           return BitmapDescriptorFactory.fromAsset(
               FlutterMain.getLookupKeyForAsset(toString(data.get(1)), toString(data.get(2))));
         }
+      default:
+        throw new IllegalArgumentException("Cannot interpret " + o + " as BitmapDescriptor");
     }
-    throw new IllegalArgumentException("Cannot interpret " + o + " as BitmapDescriptor");
   }
 
   private static boolean toBoolean(Object o) {
@@ -83,8 +84,9 @@ static CameraUpdate toCameraUpdate(Object o, float density) {
         return CameraUpdateFactory.zoomOut();
       case "zoomTo":
         return CameraUpdateFactory.zoomTo(toFloat(data.get(1)));
+      default:
+        throw new IllegalArgumentException("Cannot interpret " + o + " as CameraUpdate");
     }
-    throw new IllegalArgumentException("Cannot interpret " + o + " as CameraUpdate");
   }
 
   private static double toDouble(Object o) {

packages/google_maps_flutter/android/src/main/java/io/flutter/plugins/googlemaps/GoogleMapController.java

@@ -108,6 +108,9 @@ void init() {
       case CREATED:
         mapView.onCreate(null);
         break;
+      default:
+        throw new IllegalArgumentException(
+            "Cannot interpret " + activityState.get() + " as an activity state");
     }
     registrar.activity().getApplication().registerActivityLifecycleCallbacks(this);
     mapView.getMapAsync(this);
@@ -164,6 +167,7 @@ public void onMapReady(GoogleMap googleMap) {
     googleMap.setOnMarkerClickListener(this);
   }
 
+  @Override
   public void onMethodCall(MethodCall call, MethodChannel.Result result) {
     switch (call.method) {
       case "map#waitForMap":