Migrate to an API that exposes hot restart and hot reload operations as two phases (begin and end)

We are going to expose the two phases of the hot reload and hot restart operations in the embedder. This will make it easier to support the variety of integrations we have. We should migrate to depend on:

* `dartDevEmbedder.hotReloadBegin()`
* `dartDevEmbedder.hotReloadEnd()`
* `dartDevEmbedder.hotRestartBegin()`
* `dartDevEmbedder.hotRestartEnd()`