R1:66f20595e4d2

Start buildables in "PREPARING", move them to "BUILDING" after builds queue

Summary:
Depends on D19064. Ref T13054. See that task for additional discussion.

When buildables are created by `arc` and have lint/unit messages, they can currently pass or fail before Herald triggers actual builds. This puts them in a pre-build state where they can't complete until Herald says it's okay.

On its own, this change intentionally strands `arc diff --only` diffs in the "PREPARING" stage forever.

Test Plan:
- Ran…