Send VPN connection state metrics

Notifies that a VPN connection/disconnection event has occurred.
And gathers the current VPN state information from internal
fields and reports it to the system's statistics logging service.

Bug: 306313287
Test: atest FrameworksVpnTests
Test: statsd_testdrive 851
Flag: android.net.platform.flags.collect_vpn_metrics
Change-Id: I8d703eedb9c1b90c9c0e030dd2f6e25b1ee76a8c