Fixes ad rewards not updated at brave://rewards due to loss of precision - 1.25.x

[tmancey]

Uplift of #8786
Resolves brave/brave-browser#15801

• You have checked CI and the builds, lint, and tests all pass or are not related to your PR.
• You have tested your change on Nightly.
• The PR milestones match the branch they are landing to.

After you merge:

• The associated issue milestone is set to the smallest version that the changes is landed on.

[kjozwiak]

post-init failed due to several warning(s) via audit-deps as per https://ci.brave.com/job/pr-brave-browser-issues-15801-1.25.x-post-init/3/execution/node/145/log/. However, this is a known issue and unrelated to this specific PR.

test-browser failed on Linux as per https://ci.brave.com/job/pr-brave-browser-issues-15801-1.25.x-linux/3/testReport/. However, this is a known intermittent failure and unrelated to this specific PR.

linux / test-browser / BraveAdsBrowserTest/BraveAdsUpgradeBrowserTest.UpgradePath/PreferencesForVersion062WithRewardsEnabled_ForSupportedLocale_ForNewlySupportedLocale_RewardsShouldBeEnabled_AdsShouldBeDisabled
linux / test-browser / BraveAdsBrowserTest/BraveAdsUpgradeBrowserTest.UpgradePath/PreferencesForVersion072WithRewardsAndAdsEnabled_ForSupportedLocale_ForUnsupportedLocale_RewardsShouldBeEnabled_AdsShouldBeEnabled

[kjozwiak]

Uplift into 1.25.x approved after deliberating with @brave/uplift-approvers. QA has verified the PR on Nightly as per #8786 (comment).