Keep only one CMD_EVALUATE_PRIVATE_DNS in the queue

If a user changes the private DNS setting while NetworkMonitor
are in EvaluatingPrivateDnsState, new failure caused by
recongiguration will put into the message queue. State machine
will get multiple CMD_EVALUATE_PRIVATE_DNS events with different
delay timer which may not work as expected.

Test: atest NetworkStackTests:\
com.android.server.connectivity.NetworkMonitorTest
Bug: 139797781

Change-Id: I7f4e5642b9734daddfefc9c5ef25890aa8d3d64b