[Discounts] Add strings

Bug: b/294906968
Change-Id: I2a9b9bcf95582822601cce7b09c1f9fca2560de8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4757323


Reviewed-by: Yue Zhang <yuezhanggg@chromium.org>
Commit-Queue: Mei Liang <meiliang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180615}