Generate ninja files for api assembly and run ninja.

Change-Id: I003536e4ed4481acbdc041a6450106f8459d5978
diff --git a/orchestrator/ninja/ninja_writer.py b/orchestrator/ninja/ninja_writer.py
index e3070bb..9e80b4b 100644
--- a/orchestrator/ninja/ninja_writer.py
+++ b/orchestrator/ninja/ninja_writer.py
@@ -49,6 +49,10 @@
   def add_subninja(self, subninja: Subninja):
     self.nodes.append(subninja)
 
+  def add_phony(self, name, deps):
+    build_action = BuildAction(name, "phony", inputs=deps)
+    self.add_build_action(build_action)
+
   def write(self):
     for node in self.nodes:
       for line in node.stream():