Fix testSetOemNetworkPreferenceForTestOnlyPref flaky

There may be additional CapabilitiesChanged callbacks before
the test network is torn down. To verify ON_LOST, use
eventuallyExpect instead of expect.

Bug: 295829521
Test: atest ConnectivityManager#testSetOemNetworkPreferenceForTestOnlyPref
Change-Id: I24bb11bf2c2da08e5757c845beeb35b7004f25a4