Re-enable P3A and remove the backfill

[bsclifton]

Fixes brave/brave-browser#19302
Fixes brave/brave-browser#19324

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used GitHub auto-closing keywords in the PR description above
• 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)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• 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:

[iefremov]

p3a service lgtm

[deeppandya]

LGTM

[kjozwiak]

Verification PASSED on Pixel 6 running Android 12 using the following build

1.34.28 Chromium: 96.0.4664.45

Test Cases for brave/brave-browser#19302

Test Case #1 (Clean Install):

• downloaded/installed1.34.28 Chromium: 96.0.4664.45 and left P3A enabled via on-boarding
• double checked that P3A was enabled via Hamburger Menu -> Settings -> Brave shields & privacy
• opened several tabs & changed the default search engine via Settings
• ensured that the changes were being captured using P3A via brave://local-state

Test Case #1 (Upgrading):

• downloaded/installed 1.34.10 Chromium: 96.0.4664.45 and left P3A enabled via on-boarding
• double checked that P3A was enabled via Hamburger Menu -> Settings -> Brave shields & privacy
• opened several tabs & changed the default search engine via Settings
• ensured that the above changes weren't logged via P3A under brave://local-state
• upgraded from 1.34.28 Chromium: 96.0.4664.45
• should easily be able to tell that P3A is working/logging changes under brave://local-state by taking a look at P3A section

Test Cases for brave/brave-browser#19324

Test Case #1 (Upgrading from a broken build -> fixed build):

• downloaded/installed 1.34.10 Chromium: 96.0.4664.45 and disabled P3A via on-boarding
• double checked that P3A was disabled via Hamburger Menu -> Settings -> Brave shields & privacy
• upgraded from 1.34.10 Chromium: 96.0.4664.45 -> 1.34.28 Chromium: 96.0.4664.45
• ensured that P3A was still disabled via Hamburger Menu -> Settings -> Brave shields & privacy
• ensured that P3A entries are appearing under brave://local-state but "sent:" false (shouldn't be sending data is it's disabled)
• enable P3A via Hamburger Menu -> Settings -> Brave shields & privacy
• ensure that some of the data is eventually sent by making sure that "sent:" true

Test Case #2 (Upgrading from fixed build -> fixed build):

• downloaded/installed 1.34.19 Chromium: 96.0.4664.45 and disabled P3A via on-boarding
• double checked that P3A was disabled via Hamburger Menu -> Settings -> Brave shields & privacy
• upgraded from 1.34.19 Chromium: 96.0.4664.45 -> 1.34.28 Chromium: 96.0.4664.45
• ensured that P3A was still disabled via Hamburger Menu -> Settings -> Brave shields & privacy
• ensured that P3A entries are appearing under brave://local-state but "sent:" false (shouldn't be sending data is it's disabled)
• enable P3A via Hamburger Menu -> Settings -> Brave shields & privacy
• ensure that some of the data is eventually sent by making sure that "sent:" true

Test Case #3 (multiple upgrades):

• downloaded/installed 1.34.10 Chromium: 96.0.4664.45 and disabled P3A via on-boarding
• double checked that P3A was disabled via Hamburger Menu -> Settings -> Brave shields & privacy
• upgraded from 1.34.10 Chromium: 96.0.4664.45 -> 1.34.19 Chromium: 96.0.4664.45
• double checked that P3A was disabled via Hamburger Menu -> Settings -> Brave shields & privacy
• upgraded from 1.34.19 Chromium: 96.0.4664.45 -> 1.34.28 Chromium: 96.0.4664.45
• ensured that P3A was still disabled via Hamburger Menu -> Settings -> Brave shields & privacy
• ensured that P3A entries are appearing under brave://local-state but "sent:" false (shouldn't be sending data is it's disabled)