Use a different string for vpn summary.

- for legacy vpn config, we do not get the vpn name to show in the
preference summary, and we used to simply show "Connected" as the summary
text. However, the string was changed to include the connected device
name as the parameter. Change to use connected summary string that does
not requires any parameter.

Change-Id: Ia6191eb315f5f23e0e6bf8da2a9537c211e8188e
Fixes: 77618408
Test: make RunSettingsRoboTests