[iOS] Reduce calls to properties in PromoStyleViewController

In order to avoid repeated calls to properties and avoid code bloat,
this CL converts some private properties of PromoStyleViewController
to ivars, and cache some repeated properties calls with local variables.

Bug: 1447014
Change-Id: Ied1b4b6ada12bd2def8a75f2027998d9d9a7204b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4705206
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Menghan Yang <myuu@google.com>
Cr-Commit-Position: refs/heads/main@{#1173506}