Set P3A enabled for existing users

[deeppandya]

Resolves brave/brave-browser#13871

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:

[SergeyZhukovsky]

++

[AlexeyBarabash]

Looks good

[srirambv]

Verification passed on OnePlus 6T with Android 10 running 1.21.41 x64 Nightly build

• Verified upgrading from 1.20.39 (Nightly) to 1.21.39(Nightly) shows the P3A modal but is not enabled by default
• Background the app and relaunch shows P3A setting disabled under Privacy settings
• Verified upgrading from 1.20.39 (Nightly) to 1.21.41(Nightly) shows the P3A modal and is enabled by default
• Background the app and relaunch shows P3A setting enabled under Privacy settings