Fix farbled user agent on mobile

[pilgrim-brave]

Resolves brave/brave-browser#14740

Logic within upstream UA building code appends " Mobile" to the UA at a specific place if kUseMobileUserAgent switch is on, which it is for the Android build. This confused our test for whether the UA had been overridden by something we wanted to respect (like a command line flag or a user-modifiable runtime flag), resulting in never farbling the UA on Android. This fix replicates the kUseMobileUserAgent logic within BraveContentBrowserClient so that we properly minimize/farble the UA on all platforms, while still respecting relevant commandline/runtime flags.

Includes a new test to ensure that we are sending our minimized UA over the wire when kUseMobileUserAgent switch is on. (For Android, this means no build number or model name.)

Also migrates UA farbling tests to EvalJs.

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:

[kjozwiak]

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

1.25.31 Chromium: 90.0.4430.72

1.22.71 Chromium: 89.0.4389.114 (Release)	1.25.31 Chromium: 90.0.4430.72