WirelessNetworkSettings: add test cases for adding secure networks

Add additional test cases for adding networks with all possible
security options.

Also:
- Add a new SLEEP_TIME constant, which is shorter than TIMEOUT,
  for use in calls to Thread.Sleep(). This reduces test time.
- Clean up scrollToObject so that return conditions are more explicit.

BUG: 26883979
Change-Id: I908af1c643d61d0ff60a18520e494d52959f6e16
TEST: The added test passes on an angler.