[flutter_tools] eagerly set asset directory path, cache flutter views, simplify error handling (#68978)

Performs some small cleanup on the hot reload code path.

- Combines nested try/catch into single try catch, update on clause now that package:vm_service is used and Map does not need to be caught.
- Cache FlutterViews for the lifetime of the hot reload method handler
- Set asset directory path once during startup and remove conditional set during hot reload