Collect vpn connection metrics

Record VPN type, profile type, MTU, and allowed algorithms as
metrics when setting up a VPN connection.

Bug: 306313287
Test: atest FrameworksVpnTests
Flag: com.android.server.connectivity.collect_vpn_metrics
Change-Id: I6d009f3fdb4b0018107d76eb1b09a0bb7e8dbea9