Task/Issue URL:
https://app.asana.com/1/137249556945/project/608920331025315/task/1216118716787351?focus=true
Tech Design URL (if applicable): 

### Description
Introduces a trampoline activity for externally-triggered data clearing,
namely, for App Shortcuts.

If there are no problems with this approach, will do a follow-up to
remove the FF and legacy data clearing paths.

### Steps to test this PR

Follow steps in [Testing
instructions](https://app.asana.com/1/137249556945/task/1216212828806092?focus=true)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches full data-clearing and process restart behavior for app
shortcuts, but logic is largely duplicated from BrowserActivity with a
rollback path via feature flag; primary change is reducing
intent-redirection risk on exported BrowserActivity.
> 
> **Overview**
> Routes the **Clear Data** launcher shortcut through a new
**`FireShortcutTrampolineActivity`** (`exported=false`, translucent, no
history) instead of launching **`BrowserActivity`** with
**`PERFORM_FIRE_ON_ENTRY_EXTRA`**, so external apps cannot trigger a
full clear via the exported browser entry point.
> 
> **`AppShortcutCreator`** points dynamic shortcuts at the trampoline
when **`useFireAppShortcutTrampoline`** is on (default); otherwise it
keeps the legacy **`BrowserActivity`** intent.
**`BrowserActivity.processIntent`** ignores
**`PERFORM_FIRE_ON_ENTRY_EXTRA`** when that flag is enabled. The
trampoline runs the same manual/legacy clearing paths as before and can
delegate back to **`BrowserActivity`** if the kill switch is off (e.g.
old pinned shortcuts). Remote config adds
**`useFireAppShortcutTrampoline`** on **`AndroidBrowserConfigFeature`**.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
d38b696bb8316f842f1b80b8a2791717b42ce133. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

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Co-authored-by: Your Name <1336281+CDRussell@users.noreply.github.com>