master - Fix tcsPurchase Bugs

This commit fixes bugs surrounding tcsPurchase in the Andorid
InAppBillingImplementation class.

1. HandleActivityResult uses null-conditional operator to access
tcsPurcahse to avoid NullReferenceException.
2. HandleActivityResult now invokes TrySetException rather than
SetException to avoid InvalidOperationException.
3. ConnectAsync() now invokes TryCancel() and sets tcsPurcahse to null.
This was done to prevent tcsPurcahse from being stuck in a
WaitingForActivation TaskStatus state when the activity is destroyed in
recreated while the purchase modal is being presented to the user. This
was causing all future purchase attempts to return null when invoking
PurchaseAsync().

src/Plugin.InAppBilling.Android/InAppBillingImplementation.cs

@@ -365,6 +365,8 @@ async Task<Purchase> PurchaseAsync(string productSku, string itemType, string pa
         public override Task<bool> ConnectAsync(ItemType itemType = ItemType.InAppPurchase)
         {
             serviceConnection = new InAppBillingServiceConnection(Context, itemType);
+            tcsPurchase?.TrySetCanceled();
+            tcsPurchase = null;
             return serviceConnection.ConnectAsync();
         }
 
@@ -522,38 +524,38 @@ public static void HandleActivityResult(int requestCode, Result resultCode, Inte
 
             switch (responseCode)
             {
-                case 0:
-                    //Reponse returned OK
-                    var purchaseData = data.GetStringExtra(RESPONSE_IAP_DATA);
-                    var dataSignature = data.GetStringExtra(RESPONSE_IAP_DATA_SIGNATURE);
-
-                    tcsPurchase?.TrySetResult(new PurchaseResponse
-                    {
-                        PurchaseData = purchaseData,
-                        DataSignature = dataSignature
-                    });
-                    break;
-                case RESPONSE_CODE_RESULT_USER_CANCELED:
-                    tcsPurchase.SetException(new InAppBillingPurchaseException(PurchaseError.UserCancelled));
-                    break;
-                case RESPONSE_CODE_RESULT_SERVICE_UNAVAILABLE:
-                    tcsPurchase.SetException(new InAppBillingPurchaseException(PurchaseError.ServiceUnavailable));
-                    break;
-				case BILLING_RESPONSE_RESULT_ITEM_UNAVAILABLE:
-					tcsPurchase.SetException(new InAppBillingPurchaseException(PurchaseError.ItemUnavailable));
-					break;
-				case BILLING_RESPONSE_RESULT_DEVELOPER_ERROR:
-					tcsPurchase.SetException(new InAppBillingPurchaseException(PurchaseError.DeveloperError));
-					break;
-				case BILLING_RESPONSE_RESULT_ITEM_ALREADY_OWNED:
-					tcsPurchase.SetException(new InAppBillingPurchaseException(PurchaseError.AlreadyOwned));
-					break;
-				case BILLING_RESPONSE_RESULT_ITEM_NOT_OWNED:
-					tcsPurchase.SetException(new InAppBillingPurchaseException(PurchaseError.NotOwned));
-					break;
-				default:
-                    tcsPurchase.SetException(new InAppBillingPurchaseException(PurchaseError.GeneralError, responseCode.ToString()));
-                    break;
+	            case 0:
+		            //Reponse returned OK
+		            var purchaseData = data.GetStringExtra(RESPONSE_IAP_DATA);
+		            var dataSignature = data.GetStringExtra(RESPONSE_IAP_DATA_SIGNATURE);
+
+		            tcsPurchase?.TrySetResult(new PurchaseResponse
+		            {
+			            PurchaseData = purchaseData, DataSignature = dataSignature
+		            });
+		            break;
+	            case RESPONSE_CODE_RESULT_USER_CANCELED:
+		            tcsPurchase?.TrySetException(new InAppBillingPurchaseException(PurchaseError.UserCancelled));
+		            break;
+	            case RESPONSE_CODE_RESULT_SERVICE_UNAVAILABLE:
+		            tcsPurchase?.TrySetException(new InAppBillingPurchaseException(PurchaseError.ServiceUnavailable));
+		            break;
+	            case BILLING_RESPONSE_RESULT_ITEM_UNAVAILABLE:
+		            tcsPurchase?.TrySetException(new InAppBillingPurchaseException(PurchaseError.ItemUnavailable));
+		            break;
+	            case BILLING_RESPONSE_RESULT_DEVELOPER_ERROR:
+		            tcsPurchase?.TrySetException(new InAppBillingPurchaseException(PurchaseError.DeveloperError));
+		            break;
+	            case BILLING_RESPONSE_RESULT_ITEM_ALREADY_OWNED:
+		            tcsPurchase?.TrySetException(new InAppBillingPurchaseException(PurchaseError.AlreadyOwned));
+		            break;
+	            case BILLING_RESPONSE_RESULT_ITEM_NOT_OWNED:
+		            tcsPurchase?.TrySetException(new InAppBillingPurchaseException(PurchaseError.NotOwned));
+		            break;
+	            default:
+		            tcsPurchase?.TrySetException(
+			            new InAppBillingPurchaseException(PurchaseError.GeneralError, responseCode.ToString()));
+		            break;
             }
         }