title: PlatformChecks.RequestPermissions is not available
body: <p>I am using xamamrin for developing apps for android using c#, the IDE I am using is Visual studio.
  I want to use zxing in my app, so I add zxing packet to both core and ui layers.</p>
  
  <p>As you see in the below code, I can instantiate some classes, but I do not have any access to </p>
  
  <pre><code> PlatformChecks.IsPermissionInManifest
  </code></pre>
  
  <p>I need it to check weather the required permissions are granted or not. Please have a look at the error message I am getting</p>
  
  <p>Please let me know how to use it.</p>
  
  <p><strong>error</strong></p>
  
  <pre><code>Error CS0103: The name 'PlatformChecks' does not exist in the current context (CS0103) (MITScan.UI.Droid)
  </code></pre>
  
  <p><strong>code</strong>:</p>
  
  <pre><code> public static readonly string[] RequiredPermissions = new[] {
          Android.Manifest.Permission.Camera
      };
      protected override async void OnCreate(Android.OS.Bundle bundle)
      {
          base.OnCreate(bundle);
          this.Window.AddFlags(WindowManagerFlags.KeepScreenOn);
          SetContentView(Resource.Layout.ZxingLibTestView);
          _scannerFragment = new ZXingScannerFragment();
  SupportFragmentManager.BeginTransaction().Replace(Resource.Id.tscan_4_view_relativelayout_cameraPreview, _scannerFragment, "Scannerfragment").Commit();
  
          var permissionsToRequest = new List&lt;string&gt;();
  
          // Check and request any permissions
          foreach (var permission in RequiredPermissions)
          {
              if (PlatformChecks.IsPermissionInManifest(this, 
    permission))
              {
                  if (!PlatformChecks.IsPermissionGranted(this, 
    permission))
                      permissionsToRequest.Add(permission);
              }
          }
  
          if (permissionsToRequest.Any())
          {
              _waitingForPermission = 
    PlatformChecks.RequestPermissions(this, 
    permissionsToRequest.ToArray(), 101);
          } 
  
          }
  </code></pre>
  
  <p><strong>Note</strong>:</p>
  
  <pre><code>I am using Zxing for scanning bar codes.
  </code></pre>