Intruduce VpnConnectivityMetrics

Initial a file for VPN_CONNECTION_REPORTED metrics, which is
used to collect VPN connection related metrics.

Bug: 306313287
Test: atest FrameworksVpnTests
Flag: com.android.server.connectivity.collect_vpn_metrics
Change-Id: Iaf6c3c33e1c1ca1eff0677d8464253e78e27d3c6