Add check for min SDK when building

Author: aurimasc

com.unity.mobile.notifications/Editor/AndroidNotificationPostProcessor.cs

@@ -15,11 +15,25 @@ public class AndroidNotificationPostProcessor : IPostGenerateGradleAndroidProjec
 
         public void OnPostGenerateGradleAndroidProject(string projectPath)
         {
+            MinSdkCheck();
+
             CopyNotificationResources(projectPath);
 
             InjectAndroidManifest(projectPath);
         }
 
+        private void MinSdkCheck()
+        {
+#if !UNITY_2020_2_OR_NEWER
+        // API level 21 not supported since 2020.2, need to check for prior releases
+        const AndroidSdkVersions kMinAndroidSdk = AndroidSdkVersions.AndroidApiLevel21;
+
+        if (PlayerSettings.Android.minSdkVersion < AndroidSdkVersions.AndroidApiLevel21)
+                throw new NotSupportedException(string.Format("Minimum Android API level supported by notifications package is {0}, your Player Settings have it set to {1}",
+                    (int)kMinAndroidSdk, PlayerSettings.Android.minSdkVersion));
+#endif
+        }
+
         private void CopyNotificationResources(string projectPath)
         {
             // The projectPath points to the the parent folder instead of the actual project path.