Make UniqueNumberGenerator::AddKnownId() return a value

Make AddKnownId() return a value to indicate whether the ID was
known before, or has only been made known now.
This allows users of the class to RTC_DCHECK that no collisions
existed in their seed set, for instance.

This change is done for the following classes:
1. UniqueNumberGenerator
2. UniqueRandomIdGenerator
3. UniqueStringGenerator

Bug: None
Change-Id: I627d2821cb76aa333075e36575088d76dbeb3665
Reviewed-on: https://webrtc-review.googlesource.com/c/121780
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Amit Hilbuch <amithi@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26621}