Use components for core browsing functionality: session and engine

[mcomella]

original: https://github.com/mozilla-mobile-skunkworks/firefox-connect/issues/526

Why/User Benefit/User Problem

We want to minimize the maintenance burden of our app and much of the code we need to maintain is already handled for us in in components. Less code for us to write means more features for users. :) Additionally, the components team can even write features for us

Focus and Fire TV have migrated to components: we should too before we add too much new code that makes it hard to migrate.

Ideally, we write the tests before implementing this – #525 – so we are confident we haven't broken anything in this drastic refactor.

What / Requirements

• Integrate session component
• Integrate engine component
• See if toolbar integrates with ^ these other components automatically
• ? Verify telemetry is still working correctly
• Look into using session repo from Fire TV & potentially sharing with android components

Acceptance Criteria (how do I know when I’m done?)

• Code is using new session & engine components
• Code passes the tests from #525
• Code passes manual QA

btw, I have a work in progress (broken but browses, no tests) in https://github.com/mozilla-mobile-skunkworks/firefox-connect/tree/mcomella/i493-gv