Convert HealthKit integration code to async

[theospears]

This makes a number of changes to the HealthKit integration code. Most notably:

• All code directly working with HealthKit now uses async/await instead of callbacks. This makes control flow easier to follow, and fixes a number of bugs where success/failure callbacks were being called multiple times.
• Centralizes permission requesting inside HealthStoreManager
• Removes callback code from the path used by AppDelegate to increase the likelihood we successfully process background delivered events.

Test Plan: Have launched app, and checked manual sync buttons work for a variety of goals.

Will be running a build with these changes on my device for a few days without merging to test background sync behavior.