Add a config to set Dun capabilities.

Since CDMA doesn't use APN settings there was no place to say what a cdma
device's DUN connection would support, so by default normal device
originating traffic would be blocked on a tethering single-connection device.

With this change you can (via overlay) say that it supports everything
so mms and on-device browsing/email will still work even when on a dun connection.

The reason to allow both: some carriers will charge per byte for dun access
and so they don't want lots of non-tethering traffic used (costs the user alot)
but other carriers just use a dun connection to limit access to tethering, but
once there give unlimited data, so it makes sense to support everything there.

bug:5972599
Change-Id: I78fd7f3ac63c51a0560b659ed5ec219b10a93f8d