Show notification when failing to turn on always-on vpn

- Notification icon is Settings icon
- VpnName is obtained as the same way as AppPreference
- Post notification as the current user with PendingIntent to VpnSettings to parent user
- Auto-cancel when user taps on the notification
- This implementation posts notification only when the failure happens sychronously
  (Assume ConnectivityService only unset always-on package immediately after calling setAlwaysOnVpnPackageForUser)

Bug:27374485
Change-Id: I0aee38498c8cc300dd8eb9687adcae5f9dc4f8af