Enable Greaselion features only after rules are ready

[emerick]

Resolves brave/brave-browser#17914

Don't enable Greaselion features until Greaselion signals that all of its rules are downloaded.

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:

See STR in brave/brave-browser#17914.

[emerick]

CI passed on all platforms except Mac; restarting Mac.

[emerick]

Mac passed CI; ready to merge.

[stephendonner]

Verified PASSED using

Brave	1.31.33 Chromium: 93.0.4577.63 (Official Build) nightly (x86_64)
Revision	ff5c0da2ec0adeaed5550e6c7e98417dac77d98a-refs/branch-heads/4577@{#1135}
OS	macOS Version 11.5.2 (Build 20G95)

Steps:

1. clean profile
2. launched Brave
3. clicked on Start using Rewards
4. created a 2nd profile (but didn't enable Ads)
5. relaunched Brave
6. chose my original profile with Ads enabled
7. went to beta.talk.brave.com and clicked Start call

Confirmed my Ads-enabled state was detected and I was able to make a Brave Talk call.

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