Enables bg playback flag on Android by default

[SergeyZhukovsky]

Resolves brave/brave-browser#38869

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• 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 wiki
  • npm run presubmit wiki, 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/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Install Brave
2. Open Settings->Media
3. Make sure Background playback option is on.

[mkarolin]

++

[kjozwiak]

Verification PASSED on Pixel 6 Pro running Android 14 using the following build(s):

Brave | 1.69.13 Chromium: 126.0.6478.56 (Official Build) canary (64-bit)
--- | ---
Revision | 1565a6462a0191b7cd42f4f893be370c7039c784
OS | Android 14; Build/AP21.240305.006; 34; REL

Went through the STR/Cases mentioned via #10408 (comment) and changed the URL to https://variations.bravesoftware.com/seedFake so it doesn't pull the seed on startup.

kjozwiak@kjozwiak-VMUbuntu:~$ adb shell "echo 'https://variations.bravesoftware.com/seedFake' > /data/local/tmp/brave-test-variations-server-url"
* daemon not running; starting now at tcp:5037
* daemon started successfully

• launched Brave and ensured that brave://version doesn't list any variation studies due to using the incorrect URL
• ensured that Background playback is enabled by default as per Enables bg playback flag on Android by default #24037 (comment)
• ensured that YT clips/videos are being played in the background without any issues

Example	Example	Example