Clear VPN after Clear credentials if VPN is being established

1. There is no way to disconnect VPN after 'Clear credentials'
without removing whole Internet connection because VPN profile to
disconnect is removed when Clear credentials.

2. This commit checks whether VPN is being established or not when
Clear credentials and disconnect VPN if VPN is being established.
Lastly, this shows a toast message to inform VPN disconnected.

Test: manual - took a photo

Signed-off-by : Sungmin Lee <insight.lee@lge.com>
Bug: 29093779

Change-Id: Id5ea01c8731b3b0fca2a31d9d84e8c103952b377