[Android TRM] V2 Model Training: Allow negative request_id.

Earlier CLs (crrev.com/c/5598499 and crrev.com/c5598515) to support V2
model training feedback path (via calls to RecordAction()) required
|request_id| to be non-negative. This restriction turned out to be
extraneous, and caused missed calls to RecordAction(). This CL fixes the
issue by removing the constraints.

Also update TabResumptionModuleMediatorUnitTest to specify |requestId|
values that may be negative. Replacing |requestId| generation with
specification via List in addFakeTrainingInfo().

Bug: 344949868
Change-Id: Iafc6fd7555586eee13084a57b6d97705bff650b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5611754


Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312769}