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 a build with bin/harbormaster, saw it build normally.
• Ran a build with web UI, saw it build normally.
• Ran a build with arc diff, saw it build normally.
• Ran a build with arc diff --only, saw it hang in "PREPARING" forever.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13054

Differential Revision: https://secure.phabricator.com/D19065