Fix #1689: Display the correct BAT value for one-time-tips

[kylehickinson]

Also fixes the duplication of "USD" in the tips list

Summary of Changes

This pull request fixes issue #1689

Submitter Checklist:

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

Test Plan:

• Enable rewards
• Add a recurring tip for a publisher
• Add a one time tip for a publisher
• Verify in the rewards settings → tips details that the list displays all BAT values as regular decimals amounts (i.e. "1.0 BAT")
• Verify that USD is not repeated

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).