Add toString() for VpnProfileState

Sample log:
{State: DISCONNECTED, SessionId: null, Always-on: false,
Lockdown: false}
{State: CONNECTED, SessionId: d7913298-c07f-49de-b84e-304b5e26da89,
Always-on: true, Lockdown: false}

Bug: 225010642
Test: Add a log in Vpn.java and print it.
Change-Id: Ia36d794446114c0c2c27e725cba6a4dc1240ae38