TetheringTest has several tests that rely on starting Wifi tethering,
but do not mock WifiManager.startTetheredHotspot. This causes the
pending Wifi request to be removed, even though the test continues
thinking that the call succeeded and continues mocking Wifi AP events.
Fix this by making sure the mock returns true in setUp(), and only set
it to false when we expect that the call should fail.
Bug: 216524590
Test: atest TetheringTest
Change-Id: Iaf076b9ae64d737a8591c7f41a5552757b47999b
db7fdac6d9