Fixes issue with ads per day incorrectly limited to ads per hour

[tmancey]

Resolves brave/brave-browser#13215

Submitter Checklist:

• There is a ticket for my issue.
• Used Github auto-closing keywords in the commit message.
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed).
• Requested a security/privacy review as needed.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

See brave/brave-browser#13215

[moritzhaller]

LGTM!

[moritzhaller]

migration path LGTM!

[kjozwiak]

Reproduced the original issue on macOS 11.0.1 using the following build:

Brave | 1.20.33 Chromium: 88.0.4315.5 (Official Build) nightly (x86_64)
--- | ---
Revision | d2abe4588adaeb0d66aa0a0106675be1dff12c64-refs/branch-heads/4315@{#11}
OS | macOS Version 11.0.1 (Build 20B29)

Set Maximum number of ads displayed to 2 and went through the following:

• installed 1.20.33 CR: 88.0.4315.5 and viewed an ad (received without issues)
• waited ~30mins and viewed another ad without any issues
• waited ~30mins and never received an ad due to hitting the daily limit

Verification PASSED on macOS 11.0.1 using the following build:

Brave | 1.20.36 Chromium: 88.0.4315.5 (Official Build) nightly (x86_64)
--- | ---
Revision | d2abe4588adaeb0d66aa0a0106675be1dff12c64-refs/branch-heads/4315@{#11}
OS | macOS Version 11.0.1 (Build 20B29)

Went through the same STR mentioned above and ensured that the third ad appears in the second hour.

Example	Example