Brave Rewards notification not reflecting processed 'Auto Contribute' amount correctly

[masparrow]

Fixes brave/brave-browser#7185

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. 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-Protection-Mode
  • 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-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

As per brave/brave-browser#7185

Or;

• Clean install Brave
• Launch using command line '--rewards=reconcile-interval=3' to fast track AC's
• Enable Rewards
• Verify Uphold
• Visit Verified Publisher(s)
• Confirm sites in AC list
• Wait for and then view the contribution notification

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

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.

[tmancey]

LGTM++

[LaurenWags]

Verified auto contribute notification shows accurate contribution amount using the following build:

Brave	1.4.60 Chromium: 79.0.3945.117 (Official Build) dev (64-bit)
Revision	04f0a055010adab4484f7497fbfdbf312c307f1d-refs/branch-heads/3945@{#1019}
OS	macOS Version 10.14.6 (Build 18G103)

Steps used:

1. Clean profile 1.4.60, launch with staging flags and short reconcile interval
2. Enable rewards, claim UGP grant
3. Set AC budget to 15 BAT
4. Populate AC table with verified and non-verified sites
5. Wait for AC to complete
6. Confirm AC notification shows expected AC contribution amount, not 0.0