TfaPluginManager should use ConfigFactory not ImmutableConfig

Problem/Motivation

The TFAPluginManager stores config as an ImmutableConfig into a property.

This is an anti-pattern causing a read of 'tfa.settings' when it may not be needed and prevents updates to config after the service is initialized from being readable.

Steps to reproduce

Kernel Test:

    $this->assertArrayNotHasKey('tfa_test_plugins_send_false', $tfa_pm->getSendDefinitions(TRUE));
    $this->config('tfa.settings')
      ->set('send_plugins', ['tfa_test_plugins_send_false' => 'yo'])
      ->save();
    $this->assertArrayHasKey('tfa_test_plugins_send_false', $tfa_pm->getSendDefinitions(TRUE));

Proposed resolution

Move config_factory to a property and cease use of $tfaSettings.

Remaining tasks

User interface changes

None

API changes

Internal only class.

Data model changes

None