Add WinBackPromptEvaluator to re-engage users who were previously default browser

[catalinradoiu]

Task/Issue URL:https://app.asana.com/1/137249556945/project/1211724162604201/task/1213440050991462?focus=true

Description

Introduces a "win back" prompt for users who previously set DuckDuckGo as their default browser but no longer have it set as default. Key changes include:

• Added a new WinBackPromptEvaluator that shows the browser comparison screen to eligible users (those who were once the default browser but no longer are, and haven't seen the browser comparison prompt before). This evaluator is registered as a ModalEvaluator with the highest priority (1).
• Bumped RemoteMessageModalSurfaceEvaluator priority from 1 to 2 and AdditionalDefaultBrowserPromptsImpl priority from 2 to 3 to accommodate the new evaluator's priority slot.
• Consolidated wasEverDefaultBrowser tracking logic into the defaultBrowser setter in AppInstallSharedPreferences. Setting defaultBrowser = true now automatically and permanently flips wasEverDefaultBrowser to true, removing the need for callers to set it manually. wasEverDefaultBrowser is now a read-only val.
• Exposed wasEverDefaultBrowser through the UserBrowserProperties interface and its Android implementation.

Steps to test this PR

Win Back Prompt

• Clean install the app and enable the reactivateUsers and defaultBrowserWinBackPrompt feature toggles
• From the settings set the app as default browser and resume the app
• Set another app as default browser and then resume the app
• The re-engagement prompt should be displayed

UI changes

Show the browser comparison prompt when the user unsets the app as the default browser and then resumes the app:

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Note

Medium Risk
Changes modal evaluation priority and introduces a new re-engagement flow, which could affect which prompts users see and when. Also alters persistence semantics for default-browser history, so regressions would mainly impact eligibility/analytics rather than security.

Overview
Adds a new WinBackPromptEvaluatorImpl modal (priority 1) that launches the browser comparison prompt for users who previously set DDG as default but are no longer default, gated by feature toggles, onboarding completion, and “not previously shown” state.

Refactors default-browser history tracking by making AppInstallStore.wasEverDefaultBrowser read-only and permanently set via the defaultBrowser setter (call sites stop writing it directly); this is surfaced via a new UserBrowserProperties.wasEverDefaultBrowser() API.

Rebalances modal ordering by bumping RemoteMessageModalSurfaceEvaluator to priority 2 and AdditionalDefaultBrowserPromptsImpl to 3, and updates/adjusts unit tests accordingly.

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[catalinradoiu]

• Move the re-engagement prompt from the launcher activity to a separate modal evaluator #8376
• Add win-back prompt pixels and fallback to system settings if we cannot show the default browser dialog #8371
• Add WinBackPromptEvaluator to re-engage users who were previously default browser #8319 👈 (View in Graphite)
• develop

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