Update tnull to use new Invocation proto

This is a partial revert of https://crrev.com/c/2488843, since we went
from multi-request to single request and now back to multi-request.

BUG=chromium:1130070,chromium:1127071
TEST=local runs

Change-Id: I9ba45fdf2493a94926041996242c1f73b69bbc79
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/tnull/+/2511119
Tested-by: Sean Abraham <[email protected]>
Auto-Submit: Sean Abraham <[email protected]>
Reviewed-by: Jared Loucks <[email protected]>
Commit-Queue: Jared Loucks <[email protected]>
3 files changed