Fix WebView Browser Dialer recovery after network handovers

[eliotcougar]

Fixes #6228.

Network handovers reload the core on a worker thread. With WebView Browser Dialer enabled, stopping the existing WebView from that thread throws and aborts the dialer restart, leaving the connection broken until a manual restart.

Run WebView setup and teardown on the main looper while preserving synchronous completion and error propagation. Invalidate superseded lifecycle requests, clean up failed initialization, and ignore callbacks from destroyed views.

Validation:

• JVM tests, including 11 dialer regressions, and Play Store debug/release builds passed.
• Real WebView lifecycle tests passed on Android 17. Browser Dialer traffic recovered automatically across Wi-Fi ↔ cellular handovers in VPN and proxy-only modes. One VPN request was interrupted; the next succeeded without manual intervention.
• Lint reports four pre-existing NewApi errors in unchanged files.
• Not run: physical-device and root-mode checks; no suitable target was available.