[TECHNICAL] Kotlinize `PreviewTextFragment`

[JuancaG05]

PreviewTextFragment is the only preview activity which is still in Java, so we're moving it to Kotlin.

TASKS

• [x] Research (if needed)
• [x] Create branch technical/kotlinize_text_preview
• [x] Development tasks
  • [x] Properties
  • [x] Lifecycle activities
  • [x] Other functions
  • [x] TextLoadAsyncTask
  • [x] Remove legacy PreviewTextFragment
  • [x] Implement unit tests (if needed)
• [x] Code review and apply changes requested
• [x] Design test plan
• [x] QA
• [x] Merge branch technical/kotlinize_text_preview into master