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.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117244827

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

@@ -78,12 +78,12 @@ public final class DnsInjectionTest {
   }
 
   @Test
-  public void testWriteDnsTask_injectsAndWorks() throws Exception {
+  public void testWriteDnsAction_injectsAndWorks() throws Exception {
     persistActiveSubordinateHost("ns1.example.lol", persistActiveDomain("example.lol"));
     clock.advanceOneMilli();
     dnsQueue.addDomainRefreshTask("example.lol");
     when(req.getParameter("tld")).thenReturn("lol");
-    component.writeDnsTask().run();
+    component.writeDnsAction().run();
     assertNoDnsTasksEnqueued();
   }