[S58] O2.4: Migrate Tor Browser for Android from ESR68 to Fenix.

• Audit Fenix code for tracking, fingerprintability, and Tor safety features in relation to various known attacks.
  • Audit code changes since last audit for proxy bypass bugs, following our audit procedure.
  • Review all Mozilla developer documentation since the last audit for major changes that could impact our tracking and fingerprinting defenses.
  • Review all closed bugs in Mozilla's bug tracker to find changes not mentioned in the developer documentation that still affect our users and their threat model.
• Update the Tor Browser for Android codebase to use Fenix instead of Fennec.
  • Implement new UI changes as determined in O2.2.
  • Integrate tor into Fenix.
  • Integrate necessary webextensions into Fenix.
  • Rewrite patches from Fennec into Fenix.
• Implement and test migration logic to allow users to seamlessly upgrade from Tor Browser for Android based on Gecko to Tor Browser for Android based on Fenix.
• Release a new version of Tor Browser for Android based on Fenix.
• Document and report in a retrospective on the success of our migration process from Fennec to Fenix

• Rebase Tor Browser esr68 patches on top of mozilla-central
• Review FF release notes from FF69 to latest (FF75)
• Investigate new Search Engine configuration
• Decide which components of Fenix to rip out, disable, or use
• Audit Fenix code for tracking, fingerprintability, and safety features
• Implement new UI changes as determined in O2.1
• Test migration logic from Fennec to Fenix
• Release a new version of Tor Browser for Android based on Fenix
• Publish Fenix-based testing versions on Google Play