Parallel downloading option android

[deeppandya]

Resolves brave/brave-browser#38772

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:

[samartnik]

lgtm

[mkarolin]

chromium_src, strings++

[kjozwiak]

Verification PASSED on Pixel 6 running Android 15 using the following build(s):

Brave | 1.71.17 Chromium: 128.0.6613.40 (Official Build) canary (64-bit)
--- | ---
Revision | 72ebb76e3dbd76018bf00f14d10b73ef0d42f0c8
OS | Android 15; Build/AP31.240617.015; 35; REL

• ensured that Parallel Downloading is enabled by default
• ensured that Parallel Downloading is being displayed via Settings -> Downloads
• ensure that you can enable/disable the feature without any issues
  • also ensured that brave://flags#enable-parallel-downloading is displaying the correct state when enabled/disabled
• ensured that the current state (either enabled/disabled) is maintained after restarting the browser several times

Example	Example	Example	Example

Also ensured that the current state of the feature is maintained when updating via the following STR/Cases:

• downloaded/installed 1.71.15 Chromium: 128.0.6613.40
• disabled Parallel Downloading and restarted Brave (ensured that Parallel Downloading was disabled)
• upgraded 1.71.15 Chromium: 128.0.6613.40 --> 1.71.17 Chromium: 128.0.6613.40
• ensured that Parallel Downloading was still disabled
• ensured that you can re-enable without any issues/failures
  • checked and ensured that brave://flags#enable-parallel-downloading is enabled