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

core/java/android/net/VpnProfileState.java

@@ -24,6 +24,7 @@ import android.os.Parcelable;
 
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
+import java.util.StringJoiner;
 
 /**
  * Describe the state of VPN.
@@ -150,4 +151,29 @@ public final class VpnProfileState implements Parcelable {
         mAlwaysOn = in.readBoolean();
         mLockdown = in.readBoolean();
     }
+
+    private String convertStateToString(@State int state) {
+        switch (state) {
+            case STATE_CONNECTED:
+                return "CONNECTED";
+            case STATE_CONNECTING:
+                return "CONNECTING";
+            case STATE_DISCONNECTED:
+                return "DISCONNECTED";
+            case STATE_FAILED:
+                return "FAILED";
+            default:
+                return "UNKNOWN";
+        }
+    }
+
+    @Override
+    public String toString() {
+        final StringJoiner resultJoiner = new StringJoiner(", ", "{", "}");
+        resultJoiner.add("State: " + convertStateToString(getState()));
+        resultJoiner.add("SessionId: " + getSessionId());
+        resultJoiner.add("Always-on: " + isAlwaysOn());
+        resultJoiner.add("Lockdown: " + isLockdownEnabled());
+        return resultJoiner.toString();
+    }
 }