[ntp] Make sure only one promo type is displayed during NTP's lifecycle.

1. Prevents the mobile promo from showing if the default promo
has already been displayed, and vice versa.
2. Ensures that `middle_slot_promo.ts` doesn't fire
`ntp-middle-slot-promo-loaded` until a promo has rendered.
Before this change, mobile promos could render after the
event was fired, potentially causing a flicker [1].

[1]https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/resources/new_tab_page/app.ts;drc=2832d166ea1a726ba4795c43e07f841d56d00037;l=237

Change-Id: Ia051e6ca3965e3b93b6a4a76ebee81249c2a324f
Bug: 372923104
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5926133
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Paul Adedeji <pauladedeji@google.com>
Auto-Submit: Paul Adedeji <pauladedeji@google.com>
Cr-Commit-Position: refs/heads/main@{#1367878}