automatically migrate project files for Unity 6

Automatically update Assets/Plugins/Android/OneSignalConfig.androidlib/build.gradle
with the contains provided by the OneSignal SDK. This way the app
developer doesn't have to take any manual steps when upgrading to
Unity 6.

The changes to build.gradle were done in commit
d253d42

Author: jkasten2

com.onesignal.unity.android/Editor/Migration/MigrateAndroidResources.cs

@@ -0,0 +1,41 @@
+using System.IO;
+using UnityEditor;
+
+namespace OneSignalSDK {
+
+    [InitializeOnLoad]
+    sealed class MigrateAndroidResources {
+        static MigrateAndroidResources() {
+            UpdateBuildDotGradleContains();
+        }
+
+        /// <summary>
+        /// Updates Assets/Plugins/Android/OneSignalConfig.androidlib/build.gradle
+        /// with contains provided by OneSignal-Unity-SDK 5.1.13.
+        /// Includes compatibility with Unity 6, as it's Gradle version has new
+        /// requirements.
+        /// </summary>
+        private static void UpdateBuildDotGradleContains() {
+            if (!Directory.Exists(ExportAndroidResourcesStep._pluginExportPath))
+                return;
+
+            string exportedFilename = Path.Combine(
+                ExportAndroidResourcesStep._pluginExportPath,
+                "build.gradle"
+            );
+            string exportedContains = File.ReadAllText(exportedFilename);
+
+            string packageFilename = Path.Combine(
+                ExportAndroidResourcesStep._pluginPackagePath,
+                "build.gradle"
+            );
+            string packageContains = File.ReadAllText(packageFilename);
+
+            // We want to copy only when needed, otherwise it can reset file
+            // properties, such as permissions and timestamps
+            if (exportedContains != packageContains) {
+                File.Copy(packageFilename, exportedFilename, true);
+            }
+        }
+    }
+}

com.onesignal.unity.android/Editor/SetupSteps/ExportAndroidResourcesStep.cs

@@ -131,8 +131,8 @@ private void MigratePluginToAndroidlib() {
         private static readonly string _packagePath = Path.Combine("Packages", "com.onesignal.unity.android", "Editor");
         private static readonly string _androidPluginsPath = Path.Combine("Assets", "Plugins", "Android");
 
-        private static readonly string _pluginPackagePath = Path.Combine(_packagePath, _pluginName);
-        private static readonly string _pluginExportPath = Path.Combine(_androidPluginsPath, _pluginName);
+        internal static readonly string _pluginPackagePath = Path.Combine(_packagePath, _pluginName);
+        internal static readonly string _pluginExportPath = Path.Combine(_androidPluginsPath, _pluginName);
 
         private static readonly string _manifestPackagePath = Path.Combine(_pluginPackagePath, "AndroidManifest.xml");
         private static readonly string _manifestExportPath = Path.Combine(_pluginExportPath, "AndroidManifest.xml");