Run cleartoken task in background (#889)

Author: mehmetf

packages/google_sign_in/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java

@@ -7,7 +7,6 @@
 import android.accounts.Account;
 import android.app.Activity;
 import android.content.Intent;
-import com.google.android.gms.auth.GoogleAuthException;
 import com.google.android.gms.auth.GoogleAuthUtil;
 import com.google.android.gms.auth.UserRecoverableAuthException;
 import com.google.android.gms.auth.api.signin.GoogleSignIn;
@@ -28,7 +27,6 @@
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
 import io.flutter.plugin.common.PluginRegistry;
-import java.io.IOException;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -403,13 +401,31 @@ private static class PendingOperation {
 
     /** Clears the token kept in the client side cache. */
     @Override
-    public void clearAuthCache(Result result, String token) {
-      try {
-        GoogleAuthUtil.clearToken(registrar.context(), token);
-        result.success(null);
-      } catch (GoogleAuthException | IOException e) {
-        result.error(ERROR_REASON_EXCEPTION, e.getMessage(), e);
-      }
+    public void clearAuthCache(final Result result, final String token) {
+      Callable<Void> clearTokenTask =
+          new Callable<Void>() {
+            @Override
+            public Void call() throws Exception {
+              GoogleAuthUtil.clearToken(registrar.context(), token);
+              return null;
+            }
+          };
+
+      backgroundTaskRunner.runInBackground(
+          clearTokenTask,
+          new BackgroundTaskRunner.Callback<Void>() {
+            @Override
+            public void run(Future<Void> clearTokenFuture) {
+              try {
+                result.success(clearTokenFuture.get());
+              } catch (ExecutionException e) {
+                result.error(ERROR_REASON_EXCEPTION, e.getCause().getMessage(), null);
+              } catch (InterruptedException e) {
+                result.error(ERROR_REASON_EXCEPTION, e.getMessage(), null);
+                Thread.currentThread().interrupt();
+              }
+            }
+          });
     }
 
     /**