Use Async Storage to store study file

[hesyifei]

• Store StudyFile (i.e., StudyInfo & Streams) in Async Storage.
  • StudyInfo and Streams are stored separately (i.e., there isn't a Async Storage key storing StudyFile). It is because StudyInfo is usually more frequently used and tends to be smaller than Streams. Storing them separately probably could improve performance.
• Preparing for support downloading study file JSON.
• Created an error screen to show problems related to the survey file.
• Moved meta.startingQuestionIds to studyInfo.streamsStartingQuestionIds.
• Created mockCurrentStudyInfo helper function to help testing.
• Removed the confusing studyInfo parameter in setNotificationsAsync and getCurrentNotificationTimeAsync.
  • These functions will get study info directly from the getStudyInfoAsync() function.

Part of #3 and #11.