[url_launcher] Document why canLaunch can return false on Android API 30 (flutter#3040)

This PR adds extra documentation to the canLaunch method, explaining that (from Android API 30) false is returned when the package visibility is not managed properly.

Starting from Android API 30, apps should explicitly list which other apps it wants to query or interact with, by specifying them in the AndroidManifest.xml file. More information can be found in the Manage package visibility article in the Android documentation.

Author: mvanbeusekom

packages/url_launcher/url_launcher/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 5.7.2
+
+* Add API documentation explaining the [canLaunch] method returns `false` if package visibility (Android API 30) is not managed correctly.
+
 ## 5.7.1
 
 * Keep handling deprecated Android v1 classes for backward compatibility.

packages/url_launcher/url_launcher/lib/url_launcher.dart

@@ -109,6 +109,11 @@ Future<bool> launch(
 
 /// Checks whether the specified URL can be handled by some app installed on the
 /// device.
+///
+/// On Android (from API 30), [canLaunch] will return `false` when the required
+/// visibility configuration is not provided in the AndroidManifest.xml file.
+/// For more information see the [Managing package visibility](https://developer.android.com/training/basics/intents/package-visibility)
+/// article in the Android docs.
 Future<bool> canLaunch(String urlString) async {
   if (urlString == null) {
     return false;

packages/url_launcher/url_launcher/pubspec.yaml

@@ -2,7 +2,7 @@ name: url_launcher
 description: Flutter plugin for launching a URL on Android and iOS. Supports
   web, phone, SMS, and email schemes.
 homepage: https://github.com/flutter/plugins/tree/master/packages/url_launcher/url_launcher
-version: 5.7.1
+version: 5.7.2
 
 flutter:
   plugin: