Fix useBiddingSignalsPrioritization not to crash if reused without init.

Other values in BidderLazyFiller deliberately have checks so they don't
dereference a null pointer if the filler is reset, and then the lazy
accessors used, without reinitializing them with new values. It's not
clear if this is useful, but we should be consistent about this.

Also adds tests for this case, and refactors the
ContextRecycler.BidderLazyFiller* tests

Bug: 1517121
Change-Id: Iea9d6a12f893238316acf772eb180cdef476a8df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5245935
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1253919}