Added test for enabling wifi for demo user

Earlier change to resolve merge conflicts was breaking the current test
because mocked context was used to get wifi manager. Mocking wifi
manager to fix the test and added a check that setWifiEnabled is called
when wifi is not already on.

Test: adb shell am instrument -e class
com.android.server.retaildemo.RetailDemoModeServiceTest -w
com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner

Bug: 31833031
Change-Id: Idd123d50df6ee6db51417645fc877abebe555dc0