[Dataflow Streaming] [Multi Key] Prepare StreamingModeExecutionContext and StreamingWorkScheduler for multi-key execution.

.github/trigger_files/beam_PostCommit_Java_ValidatesRunner_Dataflow_Streaming.json

@@ -1,4 +1,4 @@
 {
   "comment": "Modify this file in a trivial way to cause this test suite to run!",
-  "modification":  1,
+  "modification":  2,
 }

.github/trigger_files/beam_PostCommit_Java_ValidatesRunner_Dataflow_Streaming_Engine.json

@@ -1,4 +1,4 @@
 {
   "comment": "Modify this file in a trivial way to cause this test suite to run!",
-  "modification":  1,
+  "modification":  2,
 }