Add useVectorFrozenFlag option in FoodCollector (#4552)

Project/Assets/ML-Agents/Examples/FoodCollector/Prefabs/VisualFoodCollectorArea.prefab

   myLaser: {fileID: 1900094563283840}
   contribute: 0
   useVectorObs: 0
-  useFrozenFlag: 1
+  useVectorFrozenFlag: 1
 --- !u!114 &114326390494230518
 MonoBehaviour:
   m_ObjectHideFlags: 0
   m_Width: 84
   m_Height: 84
   m_Grayscale: 0
+  m_ObservationStacks: 1
   m_Compression: 1
 --- !u!114 &4034342608499629224
 MonoBehaviour:
   myLaser: {fileID: 1307818939507544}
   contribute: 0
   useVectorObs: 0
-  useFrozenFlag: 1
+  useVectorFrozenFlag: 1
 --- !u!114 &114429222608880102
 MonoBehaviour:
   m_ObjectHideFlags: 0
   m_Width: 84
   m_Height: 84
   m_Grayscale: 0
+  m_ObservationStacks: 1
   m_Compression: 1
 --- !u!114 &7234640249101665162
 MonoBehaviour:
   myLaser: {fileID: 1898252046043334}
   contribute: 0
   useVectorObs: 0
-  useFrozenFlag: 1
+  useVectorFrozenFlag: 1
 --- !u!114 &114036270357198286
 MonoBehaviour:
   m_ObjectHideFlags: 0
   m_Width: 84
   m_Height: 84
   m_Grayscale: 0
+  m_ObservationStacks: 1
   m_Compression: 1
 --- !u!114 &3164735207755090463
 MonoBehaviour:
   myLaser: {fileID: 1779831409734062}
   contribute: 0
   useVectorObs: 0
-  useFrozenFlag: 1
+  useVectorFrozenFlag: 1
 --- !u!114 &114322691115031348
 MonoBehaviour:
   m_ObjectHideFlags: 0
   m_Width: 84
   m_Height: 84
   m_Grayscale: 0
+  m_ObservationStacks: 1
   m_Compression: 1
 --- !u!114 &5903164052970896384
 MonoBehaviour:

Project/Assets/ML-Agents/Examples/FoodCollector/Scripts/FoodCollectorAgent.cs

     public GameObject myLaser;
     public bool contribute;
     public bool useVectorObs;
+    [Tooltip("Use only the frozen flag in vector observations. If \"Use Vector Obs\" " +
+             "is checked, this option has no effect. This option is necessary for the " +
+             "VisualFoodCollector scene.")]
+    public bool useVectorFrozenFlag;
 
     EnvironmentParameters m_ResetParams;
 
             var localVelocity = transform.InverseTransformDirection(m_AgentRb.velocity);
             sensor.AddObservation(localVelocity.x);
             sensor.AddObservation(localVelocity.z);
+            sensor.AddObservation(m_Frozen);
-        sensor.AddObservation(m_Frozen);
+        else if (useVectorFrozenFlag)
+        {
+            sensor.AddObservation(m_Frozen);
+        }
     }
 
     public Color32 ToColor(int hexVal)