Fix #1773: Make ad auto-dismissal time 30 seconds

[kylehickinson]

Please use rebase merge strategy

Summary of Changes

This pull request fixes issue #1773
This pull request reverts #1703

Submitter Checklist:

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

Test Plan:

• Go through onboarding, verify my first ad remains on screen for 30s before timing out
• With ads enabled, browse and when an ad comes up, verify it remains on screen for 30s before timing out

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).