[iOS] Create traitCollectionDidChange Helper Function

Created a helper function named TraitCollectionSetForTraits, which
checks a kill-switch and optionally returns a list containing every
UITrait. This will make it easy to remedy any future issues that may
arise from registering to individual UITraits.

Fixed: 359488290
Change-Id: Iba1e0c0ac80f0bdda43d68f9c9a7357e1761b393
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5782087
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Daniel White <danieltwhite@google.com>
Cr-Commit-Position: refs/heads/main@{#1341675}