Wait a bit longer for wifi to shut down before rewriting the config

We've seen case of it taking longer than 1500ms for the wifi system to
actually shut down after the triggering settings element is written,
so extend the wait time a bit.  We've seen it take more than 1500ms
but not more than 2500ms, so that's the new heuristic.

This will of course all become happily obsolete once we start
applying restored AP definitions programmatically rather than by
filesystem-level operations.

Bug 22979342

Change-Id: I6acf1baac23d4100124093128b82abf242b11a0e