Fix stop behavior in PersistentConnectionManager.

PersistentConnectionManager tries to maintain a connection whenever
disconnected. Currently, there is no way to disconnect from this
connection as the manually unbind is not treated any differently than
any other disconnection. This change addresses this oversight by
suppressing reconnect logic on manual unbinds.

This change also fixes a callback issue where listeners were not
informed about disconnects when manually unbound.

Fixes: 237803638
Test: ObservableServiceConnectionTest#testUnbind
Test: PersistentConnectionManagerTest#testStopDoesNotReconnect
Change-Id: I0887e31754429cc02391df5783868d5c132a48eb