Rewrite StandaloneSpawnStrategy to use LocalSpawnRunner

PiperOrigin-RevId: 159221067
diff --git a/src/main/java/com/google/devtools/build/lib/exec/FilesetActionContextImpl.java b/src/main/java/com/google/devtools/build/lib/exec/FilesetActionContextImpl.java
index 0be6549..a234e8f 100644
--- a/src/main/java/com/google/devtools/build/lib/exec/FilesetActionContextImpl.java
+++ b/src/main/java/com/google/devtools/build/lib/exec/FilesetActionContextImpl.java
@@ -57,8 +57,8 @@
     }
 
     @Override
-    public Iterable<ActionContext> getActionContexts() {
-      return ImmutableList.<ActionContext>of(impl);
+    public Iterable<? extends ActionContext> getActionContexts() {
+      return ImmutableList.of(impl);
     }
 
     @Override