Add logging for VPN connection events

Adds logging to VpnConnectivityMetrics to record VPN connection
events.

Example logs:
VpnConnectivityMetrics logs (most recent first):
      2025-09-15T10:17:50.663552 - Report VPN connection event:\
      DISCONNECTED, vpnType=2, vpnProfileType=8, \
      underlyingNetworkTypes=[1], vpnNetworkIpProtocol=1, \
      serverIpProtocol=1, allowedAlgorithms=1999, mtu=1327
      2025-09-15T10:17:44.504636 - Report VPN connection event:\
      CONNECTED, vpnType=2, vpnProfileType=8, \
      underlyingNetworkTypes=[1], vpnNetworkIpProtocol=1, \
      serverIpProtocol=1, allowedAlgorithms=1999, mtu=1327

Bug: 429288223
Test: adb shell dumpsys vpn_management
Test: atest FrameworksVpnTests
Flag: EXEMPT DEBUG
Change-Id: Ic8d1f6411ade28a36d64dd2925bbd24ec8e567f1