Fix ThrottleService wifi interaction

When we switch to another network (or even just lose mobile data) the mobile
interface is either brought down or destroyed altogether.  That means any
throttling rules we've told the kernel about get lost.

This change notices when our interface comes up and reinstates throttling
if needed.

bug:2660287
Change-Id: I4c6ebf78acc59ed66b95984a1e8b7c2ffbc22dcb