[PriceInsights][iOS] Assign snackbar message based on user notification status.

This CL conditionally assigns a message to the snackbar based on
the user's notification status.
Additionally, it fixes the bug where the tracking toast message
appears at the same time as the notification prompt.

Bug: chromium:346601270
Change-Id: I14fa785870fa315cd6408fddd396dc8f40a6247f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5676392
Reviewed-by: Gayane Petrosyan <gayane@google.com>
Reviewed-by: David Maunder <davidjm@chromium.org>
Commit-Queue: Cheick Cisse <cheickcisse@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1329619}