Refactor FXIOS-11271 #24515 Lazy loading in bvc

[lmarceau]

📜 Tickets

Jira ticket
Github issue

💡 Description

Follow-up work on #24487. This won't have a huge impact on load time, but I still think it's good to try to improve here. This PR is mainly doing two things:

• Adding lazy top some of our UI elements in BrowserViewController so we load them only when needed. We were already doing that to some elements, so this doesn't have a huge impact after profiling (~10 ms faster, I'll take it 😆 ).
• Reordering declarations at the top of BrowserViewController and adding MARK so it's better organized.

Further notes:

• searchTelemetry was set as optional, when it was in fact always created in the BVC init.
• Removed unused ActionSheetTitleMaxLength static var
• Rename urlBarHeightConstraint to legacyUrlBarHeightConstraint since it's only used for the legacy URLBar
• Changed to private or private(set) some variable that could be private

📝 Checklist

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• Filled in the above information (tickets numbers and description of your work)
• Updated the PR name to follow our PR naming guidelines
• Wrote unit tests and/or ensured the tests suite is passing
• When working on UI, I checked and implemented accessibility (minimum Dynamic Text and VoiceOver)
• If needed, I updated documentation / comments for complex code and public methods
• If needed, added a backport comment (example @Mergifyio backport release/v120)

[FilippoZazzeroni]

LGTM 🎉 🎉 thanks @lmarceau to add the MARK separation it looks a lot more organized