Fix Test fails in PHP 7 due to use of reserved keyword "String"

Problem/Motivation

Tests in PHP 7 are failing due to some tests and classes using the reserved "String" keyword. See below test summary.

12:55:54 Test summary
12:55:54 ------------
12:55:54 
12:55:57 
12:55:57 Fatal error: Cannot use Drupal\Component\Utility\String as String because 'String' is a special class name in /var/www/html/modules/smsframework/modules/sms_valid/sms_valid.install on line 11
12:55:57 FATAL Drupal\sms_valid\Tests\SmsValidWebTest: test runner returned a non-zero error code (255).
12:55:57 
12:55:57 Fatal error: Cannot use Drupal\Component\Utility\String as String because 'String' is a special class name in /var/www/html/modules/smsframework/src/Form/CarrierAdminForm.php on line 11
12:55:57 
12:55:57 Fatal error: Cannot use Drupal\Component\Utility\String as String because 'String' is a special class name in /var/www/html/modules/smsframework/src/Form/CarrierAdminForm.php on line 11
12:55:57 FATAL Drupal\sms_devel\Tests\SmsDevelWebTest: test runner returned a non-zero error code (255).
12:55:57 FATAL Drupal\sms\Tests\SmsFrameworkWebTest: test runner returned a non-zero error code (255).
12:55:57 
12:55:57 Fatal error: Cannot use Drupal\Component\Utility\String as String because 'String' is a special class name in /var/www/html/modules/smsframework/src/Form/CarrierAdminForm.php on line 11
12:55:57 FATAL Drupal\sms_track\Tests\SmsTrackArchiveTest: test runner returned a non-zero error code (255).
12:55:58 
12:55:58 Fatal error: Cannot use Drupal\Component\Utility\String as String because 'String' is a special class name in /var/www/html/modules/smsframework/src/Form/CarrierAdminForm.php on line 11
12:55:58 FATAL Drupal\sms_blast\Tests\SmsBlastWebTest: test runner returned a non-zero error code (255).
12:55:59 
12:55:59 Fatal error: Cannot use Drupal\Component\Utility\String as String because 'String' is a special class name in /var/www/html/modules/smsframework/src/Form/CarrierAdminForm.php on line 11
12:55:59 FATAL Drupal\sms_user\Tests\SmsUserWebTest: test runner returned a non-zero error code (255).
12:55:59 
12:55:59 Test run duration: 5 sec

Proposed resolution

Replace use of String class, which have been removed in D8. Add tests to catch fails in PHP 5

Remaining tasks

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API changes

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Data model changes