Expand Rewards to three decimal places

[tmancey]

Summary of Changes

This pull request fixes #2596

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

See brave/brave-browser#10027

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).

[kylehickinson]

I know we're having this conversation already but just to document: Are we sure we want to always show 3 decimal places instead of a min/max number of decimals shown based on the value itself? We can easily do this with NumberFormatter

[tmancey]

^^^ @jsecretan

[tmancey]

As we always show 3 decimal places for desktop, I will merge as-is. And if a decision is made to change we should raise a new issue for all platforms