3877 [in_app_purchase] Implementation of the app facing package (cla: yes, p: in_app_purchase, platform-ios, platform-android)

3879 [in_app_purchase] Update to pub version of in_app_purchase_platform_interface dependency (cla: yes, waiting for tree to go green, p: in_app_purchase, platform-ios, platform-android)

3894 Bump min Android SDK to the version required at runtime (cla: yes, waiting for tree to go green, p: webview_flutter, p: google_maps_flutter, platform-android)

3902 [url_launcher] Migrate maven repo from jcenter to mavenCentral (cla: yes, waiting for tree to go green, p: url_launcher, platform-android)