Remove UA farbling on all platforms and unconditionally exclude Android model

[pilgrim-brave]

Resolves brave/brave-browser#15372
Resolves brave/brave-browser#15322
Resolves brave/brave-browser#15039

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:

[bridiver]

please add network UA tests back in always checking against the unfarbled UA

[pilgrim-brave]

Added new DEPS file since review, as noted by gn_check

[kjozwiak]

Verification PASSED on Win 10 x64 using the following build:

Brave | 1.25.33 Chromium: 90.0.4430.85 (Official Build) nightly (64-bit)
-- | --
Revision | 5bc145d831c180d9ff94f29a0d7a2e1cbd30ef36-refs/branch-heads/4430@{#1311}
OS | Windows 10 OS Version 2009 (Build 19042.928)

Verification PASSED on Samsung S10+ running Android 11 using the following build:

1.25.31 Chromium: 90.0.4430.72

Test Case brave/brave-browser#15039

Went through the STR/Cases outlined via brave/brave-browser#15039 (comment) and ensured that Cloudflare correctly directed/opened http://chaingames.io.

Desktop	Android

Test Case brave/brave-browser#15322

1.22.71 Chromium: 89.0.4389.114 (Release)	1.25.31 Chromium: 90.0.4430.72