testTetheringUpstream test case need cellular network. So it disable
wifi and getActiveNetwork to check whether current network is cellular.
But there is a race that call getActiveNetwork right away after
disconnect wifi. getActiveNetwork may return null because switching
network is not ready yet.
Bug: 156314879
Test: atest CtsTetheringTest
Change-Id: I6b433560fb27a3b4bd80c4198ccdb58a5be701b6
47e3788e3e