Fix AppTP unregister crash

[joshliebe]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1200204095367872/task/1216284733725748?focus=true
Tech Design URL (if applicable):

Description

• Fixes a crash when disabling AppTP

Steps to test this PR

• Enable AppTP
• Disable AppTP
• Verify AppTP is disabled without crash

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Note

Low Risk
Small lifecycle guard around VPN DNS monitoring; no change to when DNS changes trigger VPN restarts.

Overview
Fixes an AppTP crash when DnsChangeCallback calls unregisterNetworkCallback without a matching prior registration (e.g. VPN restart/stop paths that invoke unregister() more than once or when the Moto G DNS listener was never registered).

DnsChangeCallback now tracks a registered flag: register() is a no-op if already registered and only sets the flag after a successful registerNetworkCallback; unregister() skips when not registered and clears the flag after the unregister attempt (including on failure).

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[joshliebe]

• Fix AppTP unregister crash #9114 👈 (View in Graphite)
• develop

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