Add a branded VPN icon for quick settings and status bar.

If a system VPN app specifies in its meta-data that it should use the system's
branded VPN icon instead of the standard one, the Quick Settings footer and
status bar show it instead.

Bug 27146125

Change-Id: I3cfe4dda156d9147bdb339ba6bce82c72ca7393d
(cherry picked from commit 39566391)