Cleanup StudyScreen

[hesyifei]

• Rewrite the logic in the SurveyScreen (especially in the onNextSelect function) to make it clearer.
• Add many comments in the StudyScreen.
• Rename states and props in StudyScreen to make it clearer.
• Combined StudyScreen's currentQuestionId state and extraMetaData state into currentQuestionData state to make it easier for stacking future question ID and data.
• Decompose addAnswerToAnswersListAsync in SurveyScreen to insertAnswerAsync in the answers helper file.
• Add a isInTransition state in SurveyScreen that would make the screen blank before each question is loaded
• Let QuestionIdSchema allow [ and ] so that the variable placeholders can be used properly.
• Let MultipleTextQuestionSchema's indexName and variableName be question ID-compatible as they might be used in question ID.
• Add fallbackNext to BaseQuestionSchema
  • Remove "_pna" in specialCasesStartId from ChoicesQuestionSchema.
  • Remove fallbackItemStartId from MultipleTextQuestionSchema.
• Use an array of tuples instead of an object for specialCasesStartId in ChoicesQuestionSchema
  • This way the special case next question is deterministic in the case of ChoicesWithMultipleAnswers.

For tests:

• Add tests for StudyScreen.
  • Tests for branching questions not added yet.
• Fix some wrong usages of waitFor. Fix some problems with act.

Part of #11.