[WifiCoex] Fix race condition in cts test coex callback

Fix race condition in testListenOnCoexUnsafeChannels by waiting for
callback to run before unregistering the callback.

Bug: 177665950
Test: atest android.net.wifi.cts.WifiManagerTest
Change-Id: I194c5cd5f286810e2cc68da736051c444a247859
1 file changed