fix loss of data connectivity on devices that don't register their gateways in framework

some devices (namely sholes) do not use the android framework to define a default gateway.
this causes problems when the connection is put into suspension as the default gateway is
removed and never restored.

this solution caches the current gateway (determined via native call) when the device
is removing it due to suspension and the default gateway is not known to framework.
the cached gateway is then used when adding a default gateway in the event that one was
not provided in the interim.

Change-Id: I4a8a5d2e567d11689d7f70118582a92c5ee9b0c5