Rename Runnable classes from Tasks to Actions

This is cleanup relating to a naming decision that we made awhile ago
but never got around to.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117244827

javatests/com/google/domain/registry/dns/DnsTestComponent.java

@@ -32,6 +32,6 @@ import dagger.Component;
 })
 interface DnsTestComponent {
   DnsQueue dnsQueue();
-  RefreshDns refreshDns();
-  WriteDnsTask writeDnsTask();
+  RefreshDnsAction refreshDns();
+  WriteDnsAction writeDnsAction();
 }