Show correct menu icon on app startup

[joshliebe]

Task/Issue URL: https://app.asana.com/1/137249556945/project/488551667048375/task/1214187547786023?focus=true

Description

• The menu icon was recently updated but the mock omnibar/layout XML still use the old icon, which means it shows briefly on startup.

Solution

• When we bindMockupToolbars then the icon should be set based on the menu feature flag.

• The initial BrowserMenuDisplayState should be the actual state based on the feature flag instead of false.

Steps to test this PR

• Kill the app and reopen
• Verify that the correct menu icon is shown on startup

UI changes

Before	After

---

Note

Low Risk
Low risk UI-state change: adds a synchronous check for the bottom-sheet menu flag to avoid incorrect initial state before flows emit, with unit tests covering the new logic.

Overview
Ensures the browser menu icon is correct immediately on startup by introducing BrowserMenuDisplayRepository.isBottomSheetMenuEnabled() and using it to set initial useBottomSheetMenu/menu icon state before reactive flows emit.

BrowserActivity now picks the mock toolbar menu icon (hamburger vs vertical) based on this synchronous check, and BrowserTabViewModel initializes browserMenuState/BrowserViewState with the same derived value. Adds unit tests for the new enablement computation across rollout/experimental flags and user preference.

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[joshliebe]

• Show correct menu icon on app startup #8337 👈 (View in Graphite)
• develop

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[malmstein]

works as expected. LGTM