Remove special handling of SpawnActionContext in Executor/ActionExecContext

Instead, internally look up the correct context by mnemonic. This simplifies
all the callers. We still need a little bit of special casing when constructing
the action context map.

PiperOrigin-RevId: 197572357
diff --git a/src/test/java/com/google/devtools/build/lib/standalone/StandaloneSpawnStrategyTest.java b/src/test/java/com/google/devtools/build/lib/standalone/StandaloneSpawnStrategyTest.java
index d4ecd6c..da31f12 100644
--- a/src/test/java/com/google/devtools/build/lib/standalone/StandaloneSpawnStrategyTest.java
+++ b/src/test/java/com/google/devtools/build/lib/standalone/StandaloneSpawnStrategyTest.java
@@ -172,14 +172,14 @@
   @Test
   public void testBinTrueExecutesFine() throws Exception {
     Spawn spawn = createSpawn(getTrueCommand());
-    executor.getSpawnActionContext(spawn).exec(spawn, createContext());
+    executor.getContext(SpawnActionContext.class).exec(spawn, createContext());
 
     assertThat(out()).isEmpty();
     assertThat(err()).isEmpty();
   }
 
   private List<SpawnResult> run(Spawn spawn) throws Exception {
-    return executor.getSpawnActionContext(spawn).exec(spawn, createContext());
+    return executor.getContext(SpawnActionContext.class).exec(spawn, createContext());
   }
 
   private ActionExecutionContext createContext() {