Issues during admin form save

Problem/Motivation

During save of administration form few PHP warnings and notices appear (some few time per each form field triggered):

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'baidu_analytics_token_element_validate' not found or invalid function name in Drupal\Core\Form\FormValidator->doValidateForm() (line 281 of core/lib/Drupal/Core/Form/FormValidator.php).
Notice: Undefined index: #default_value in token_element_validate() (line 260 of modules/contrib/token/token.module).
Warning: Invalid argument supplied for foreach() in Drupal\baidu_analytics\Form\BaiduAnalyticsAdminSettingsForm->validateForm() (line 467 of modules/contrib/baidu_analytics/src/Form/BaiduAnalyticsAdminSettingsForm.php).

This is due to port leftovers and missing validation function from 7.x version.

Proposed resolution

Fix validation conditions form render array according to new array indexes.
Port remaining validation function to current administration form.

Remaining tasks

Review code.

User interface changes

None

API changes

None

Data model changes

None