Add AndroidProrationMode values (#273)

* Add AndroidProrationMode values
   New class AndroidProrationMode with valid values as per 
   https://developer.android.com/reference/com/android/billingclient/api/BillingFlowParams.ProrationMode

* Add detailed descriptions

lib/flutter_inapp_purchase.dart

@@ -219,7 +219,8 @@ class FlutterInappPurchase {
 
   /// Request a purchase on `Android` or `iOS`.
   /// Result will be received in `purchaseUpdated` listener or `purchaseError` listener.
-  ///
+  /// 
+  /// Check [AndroidProrationMode] for valid proration values
   /// Identical to [requestSubscription] on `iOS`.
   Future requestPurchase(
     String sku, {
@@ -250,7 +251,8 @@ class FlutterInappPurchase {
   /// Result will be received in `purchaseUpdated` listener or `purchaseError` listener.
   ///
   /// **NOTICE** second parameter is required on `Android`.
-  ///
+  /// 
+  /// Check [AndroidProrationMode] for valid proration values
   /// Identical to [requestPurchase] on `iOS`.
   Future requestSubscription(
     String sku, {
@@ -658,3 +660,26 @@ class FlutterInappPurchase {
     }
   }
 }
+
+/// A list of valid values for ProrationMode parameter
+/// https://developer.android.com/reference/com/android/billingclient/api/BillingFlowParams.ProrationMode
+class AndroidProrationMode{
+  /// Replacement takes effect when the old plan expires, and the new price will be charged at the same time.
+  /// https://developer.android.com/reference/com/android/billingclient/api/BillingFlowParams.ProrationMode#DEFERRED
+  static const int DEFERRED = 4;
+
+  /// Replacement takes effect immediately, and the billing cycle remains the same. The price for the remaining period will be charged. This option is only available for subscription upgrade.
+  /// https://developer.android.com/reference/com/android/billingclient/api/BillingFlowParams.ProrationMode#immediate_and_charge_prorated_price
+  static const int IMMEDIATE_AND_CHARGE_PRORATED_PRICE = 2;
+
+  /// Replacement takes effect immediately, and the new price will be charged on next recurrence time. The billing cycle stays the same.
+  /// https://developer.android.com/reference/com/android/billingclient/api/BillingFlowParams.ProrationMode#immediate_without_proration
+  static const int IMMEDIATE_WITHOUT_PRORATION = 3;
+
+  /// Replacement takes effect immediately, and the remaining time will be prorated and credited to the user. This is the current default behavior.
+  /// https://developer.android.com/reference/com/android/billingclient/api/BillingFlowParams.ProrationMode#immediate_with_time_proration
+  static const int IMMEDIATE_WITH_TIME_PRORATION = 1;
+
+  /// https://developer.android.com/reference/com/android/billingclient/api/BillingFlowParams.ProrationMode#unknown_subscription_upgrade_downgrade_policy
+  static const int UNKNOWN_SUBSCRIPTION_UPGRADE_DOWNGRADE_POLICY = 0;
+}