DoH: Automatically reconnect closed connections

Previously, I assumed a reprobe would be issued after a network died to
re-establish it. As this is not the case, this change automatically
re-establishes the connection to survive the stress test.

Bug: 202081046
Bug: 203314532
Test: resolv_integration_test
Test: resolv_stress_test + I682678b84b35c575a3eb88c2c1c67aefd195616c
Change-Id: I4c934429f2eb827382eccc10dea82c1cc8a78d4a