Send connection metrics when status changes

VPN metrics should send disconnected and connected events when
the underlying network or IP protocol changes.

Bug: 306313287
Test: atest FrameworksVpnTests
Test: Verify some scenarios manually:
  - Switch underlying network between Wi-Fi and cellular.
  - Switch between platform VPN and 3rd-party VPN.
  - Disable MOBIKE feature.
Flag: android.net.platform.flags.collect_vpn_metrics
Change-Id: I228fa4f6c6b6c6f217a58a1e6acf3bc295cb0d3f