[Angular] A read-only Dropdown field allows users to change its value

[JaneSjs]

T18827 - For dropdown, mode "display" is allowing to select the options https://surveyjs.answerdesk.io/internal/ticket/details/T18827

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https://github.com/surveyjs/survey-library/assets/22372972/ff29270e-a350-4cf5-9948-cbd941e42094

To reproduce the issue, insert the following JSON within a creator at https://surveyjs.io/survey-creator/examples/free-nps-survey-template/angular#.

{
 "pages": [
  {
   "name": "page1",
   "elements": [
    {
     "type": "dropdown",
     "name": "question1",
     "defaultValue": "Item 2",
     "choices": [
      "Item 1",
      "Item 2",
      "Item 3"
     ]
    }
   ]
  }
 ],
 "mode": "display"
}

[dmitry-kurmanov]

related to https://github.com/surveyjs/survey-library/issues/8394