Ensure introductoryPriceAmountMicros field to be String.

Converts the introductoryPriceAmountMicros field to a String value
before it is send as JSON string from Java to Dart. This will solve the
issue
(flutter#4364 (comment)) in
a non-breaking manor (updating the type on the Dart side would introduce
a breaking change).

Author: mvanbeusekom

packages/in_app_purchase/in_app_purchase_android/android/src/main/java/io/flutter/plugins/inapppurchase/Translator.java

@@ -26,7 +26,9 @@ static HashMap<String, Object> fromSkuDetail(SkuDetails detail) {
     info.put("description", detail.getDescription());
     info.put("freeTrialPeriod", detail.getFreeTrialPeriod());
     info.put("introductoryPrice", detail.getIntroductoryPrice());
-    info.put("introductoryPriceAmountMicros", detail.getIntroductoryPriceAmountMicros());
+    // Make sure the `introductoryPriceAmountMicros` field is returned as a `String` value as the
+    // Dart side expects it as `String`. Changing the type in Dart would introduce a breaking change.
+    info.put("introductoryPriceAmountMicros", String.valueOf(detail.getIntroductoryPriceAmountMicros()));
     info.put("introductoryPriceCycles", detail.getIntroductoryPriceCycles());
     info.put("introductoryPricePeriod", detail.getIntroductoryPricePeriod());
     info.put("price", detail.getPrice());