Prevent extension crash when omnibox API is not available.

[sammacbeth]

Reviewer:

Description:

Guards our use of browser.omnibox so the extension doesn't crash when this API is missing. This is the case in Firefox on Android and for Safari MV3 extensions.

I've also moved this code into its own component, as part of ongoing work to clean up events.js.

Steps to test this PR:

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications