Add alert dialog when always-on VPN disconnects.

As part of the improvement to always-on VPN, we're adding this dialog
which is shown when the user taps the "Always-on VPN disconnected"
notification. This dialog shows a relatively detailed explanation of the
situation and offers two actions: 1) to attempt to reconnect, and 2) to
open the VpnSettings page in Settings. As a result, we expect the users
to be more aware of the consequences of a disconnected VPN, and offer
them more actionable options.

Bug: 36650087
Test: manual

Change-Id: I5ae3ff5d25740ea52357012b75d7eb1776dfdc5e