Fix Android WebAuth callback for system browser flows (#15187)

* Fix Android WebAuth callback for system browser

This adds a flag to track if the web authentication flow was launched via custom tabs or the fallback, system browser.

We were assuming it was always via custom tabs, and so the intermediate activity would be started which is not necessary or designed to compatible with the flow using the system browser.

This changes the callback activity to properly route back to the intermediate activity for custom tabs initialized flows, and to just call the OnResume callback for other flows (system browser).

* Auto-format source code

---------

Co-authored-by: GitHub Actions Autoformatter <autoformat@example.com>

src/Essentials/src/WebAuthenticator/WebAuthenticator.android.cs

@@ -13,6 +13,8 @@ partial class WebAuthenticatorImplementation : IWebAuthenticator, IPlatformWebAu
 		Uri currentRedirectUri = null;
 		WebAuthenticatorOptions currentOptions = null;
 
+		internal bool AuthenticatingWithCustomTabs { get; private set; } = false;
+
 		public bool OnResumeCallback(Intent intent)
 		{
 			// If we aren't waiting on a task, don't handle the url
@@ -70,7 +72,11 @@ public async Task<WebAuthenticatorResult> AuthenticateAsync(WebAuthenticatorOpti
 			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;

src/Essentials/src/WebAuthenticator/WebAuthenticator.shared.cs

@@ -102,6 +102,11 @@ static IPlatformWebAuthenticatorCallback AsPlatformCallback(this IWebAuthenticat
 			return platform;
 		}
 
+#if ANDROID
+		internal static bool IsAuthenticatingWithCustomTabs(this IWebAuthenticator webAuthenticator)
+			=> (webAuthenticator as WebAuthenticatorImplementation)?.AuthenticatingWithCustomTabs ?? false;
+#endif
+
 		/// <summary>
 		/// Begin an authentication flow by navigating to the specified url and waiting for a callback/redirect to the callbackUrl scheme.
 		/// </summary>

src/Essentials/src/WebAuthenticator/WebAuthenticatorCallbackActivity.android.cs

@@ -10,11 +10,21 @@ protected override void OnCreate(Bundle savedInstanceState)
 		{
 			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.Default.IsAuthenticatingWithCustomTabs())
+			{
+				// 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.Default.OnResume(Intent);
+			}
 
 			// finish this activity
 			Finish();