NullAway: Update library model to assume findPreference() returns non-null

It so rarely does return null, that having to assume the return value is
not worth it.

Bug: 389129271
Change-Id: Ie72f21d8d163cfbe66bea0eebadcf00cfce082fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6383304
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Owners-Override: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1436767}
NOKEYCHECK=True
GitOrigin-RevId: 4986bebea5e68546946c45072c36b672dbbab6fc