[unified_analytics] Key consent and opt-in on rootTool #2701

Depends on #2700.

• Key these on rootTool instead of tool: the first-run and message-version checks (parsedTools[...]), clientShowedMessage() (the config-file write), and getConsentMessage (tool description and dart vs. flutter command).
• If no rootTool resolves, which can only happen for shared components, disable analytics for the session. That means okToSend == false, no consent message, and no config-file write.

Done when: a shared component never shows a prompt or writes its own config line. User-visible consent behavior for top-level tools stays the same.