Reset VpnConnectivityMetrics on VPN disconnect

Upon VPN disconnection, the associated NetworkAgent becomes
unregistered. To ensure that metrics collected for subsequent
VPN connections are accurate and not influenced by prior data,
the VpnConnectivityMetrics state should be reset at this point.

Bug: 306313287
Test: atest FrameworksVpnTests
Flag: android.net.platform.flags.collect_vpn_metrics
Change-Id: I95c190dc34b8b00aa46562b4cdd8dbffb8d2c961