[in_app_purchase] Convert Android data objects to Pigeon (flutter#6453)

Follow-up to flutter#6262, converting the nested data objects from the legacy JSON conversion to typed Pigeon objects.

Deprecates all the JSON code in the wrapper objects, but does not remove it because, unfortunately, the wrapper objects were both platform data transfer objects *and* the public API surface, so the JSON serialization/deserialization code that was part of those objects for platform channel usage is part of the public API as well. At some point we can do a breaking change to clean up that code (flutter/flutter#146202).

Fixes flutter/flutter#117910

Author: stuartmorgan-g

packages/in_app_purchase/in_app_purchase_android/CHANGELOG.md

@@ -1,3 +1,9 @@
+## 0.3.3
+
+* Converts data objects in internal platform communication to Pigeon.
+* Deprecates JSON serialization and deserialization for Billing Client wrapper
+  objects.
+
 ## 0.3.2+1
 
 * Converts internal platform communication to Pigeon.