Fix failing auth redirect on Android

Xamarin.Essentials/WebAuthenticator/WebAuthenticator.android.cs

@@ -14,6 +14,8 @@ namespace Xamarin.Essentials
         static TaskCompletionSource<WebAuthenticatorResult> tcsResponse = null;
         static Uri currentRedirectUri = null;
 
+        internal static bool AuthenticatingWithCustomTabs { get; private set; } = false;
+
         internal static bool OnResume(Intent intent)
         {
             // If we aren't waiting on a task, don't handle the url
@@ -71,7 +73,11 @@ namespace Xamarin.Essentials
             tcsResponse = new TaskCompletionSource<WebAuthenticatorResult>();
             currentRedirectUri = callbackUrl;
 
-            if (!(await StartCustomTabsActivity(url)))
+            // Try to start with custom tabs if the system supports it and we resolve it
+            AuthenticatingWithCustomTabs = await StartCustomTabsActivity(url);
+
+            // Fall back to using the system browser if necessary
+            if (!AuthenticatingWithCustomTabs)
             {
                 // Fall back to opening the system-registered browser if necessary
                 var urlOriginalString = url.OriginalString;

Xamarin.Essentials/WebAuthenticator/WebAuthenticatorCallbackActivity.android.cs

@@ -10,11 +10,21 @@ namespace Xamarin.Essentials
         {
             base.OnCreate(savedInstanceState);
 
-            // start the intermediate activity again with flags to close the custom tabs
-            var intent = new Intent(this, typeof(WebAuthenticatorIntermediateActivity));
-            intent.SetData(Intent.Data);
-            intent.AddFlags(ActivityFlags.ClearTop | ActivityFlags.SingleTop);
-            StartActivity(intent);
+            // Check how we launched the flow initially
+            if (WebAuthenticator.AuthenticatingWithCustomTabs)
+            {
+                // start the intermediate activity again with flags to close the custom tabs
+                var intent = new Intent(this, typeof(WebAuthenticatorIntermediateActivity));
+                intent.SetData(Intent.Data);
+                intent.AddFlags(ActivityFlags.ClearTop | ActivityFlags.SingleTop);
+                StartActivity(intent);
+            }
+            else
+            {
+                // No intermediate activity if we returned from a system browser
+                // intent since there's no custom tab instance to clean up
+                WebAuthenticator.OnResume(Intent);
+            }
 
             // finish this activity
             Finish();