[Impeller] Migrate the Embedder API to use Impeller.

[chinmaygarde]

Migrating to Impeller will not change the public Embedder API in any way. But, Impeller needs to be wired up as a backend.

[chinmaygarde]

From the Impeller Weekly sync: This would also allow us to use the embedder test harnesses on Mac as Impeller smoke tests. Elevating priority and assigning.

[chinmaygarde]

Making good progress on this. Should land this for Metal this week.