[oobe] Switch ZTE end to end test to USD

Since M131, unified enrollment state determiantion (USD) has replaced
legacy state determintation (initial and FRE). This CL switches a ZTE
integration from legacy to USD.

As part of that it also updates the device state retrieval logic in the
embedded test server to match DMServer, i.e. a single request to check
for ZTE and FRE (in that order).

Bug: 383047722
Test: Refactored browser test and new unit tests
Change-Id: I5831c7cd96b38e62f45872ec95e0e94c443b3b39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6275936
Reviewed-by: Renato Silva <rrsilva@google.com>
Reviewed-by: Artem Sumaneev <asumaneev@google.com>
Commit-Queue: Roland Bock <rbock@google.com>
Cr-Commit-Position: refs/heads/main@{#1422515}