Prevent windows overlay on VPN dialog

This change addresses the issue of some applications using tapjack
attacks to trick users into disconnecting the VPN connection.

Bug: 182806691
Test: manually check that the VPN dialog is not being overlaid by
windows from the PoC application.

Change-Id: I03e600dec306af8b7d4c7ec024a5903f4591d9b4
1 file changed