Only allow System apps to make VPN exempt routes

requestRouteToHost will only allow system applications to make routes
exempt from the VPN's routing rules.

If a VPN is currently running and a non-system app requests a route it
will only succeed if that host is currently covered by a VPN exempt
routing rule. Otherwise it will fail.

For example, if a VPN is running and the MMS network is brought online
those routes will be added as VPN exempt. If an application then tries
to request a route to a MMS endpoint it will succeed because the routes
already exist. If an application tries to request a route to a host
covered by the VPN the call will fail.

Bug: 12937545
Change-Id: If7bcec91bbb96c62c8fb69748c975847e6c00b6f
Update LinkProperties with MTU value

Use the MTU provided by network as part of Data Call Response.
If it is not present, use the static value from config file

Change-Id: I9d03cc9b355d09a93883fbf4ddc93fae0077641c
CRs-Fixed: 634747