Associated CLs are refactoring code that this API relies on, and it's
not currently covered by any tests. This CL adds such a test to
ConnectivityManagerTest.
Test: Ensured that the following passes, and that it also passes when
the early-exit (no WiFi/cellular connection) is removed:
make cts && cts-tradefed run cts -m CtsNetTestCases \
-t android.net.cts.ConnectivityManagerTest
Bug: 64021405
Change-Id: I84806533368fa592b9b8f4c78900542413af6aac
d752d833ca