gn2bp: add `get_cmd` to BaseActionSanitizer

Test: update_results.sh
Change-Id: I054607965a7011b1fccb67b5e89744b86dce2345
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 52d3be6..6686f99 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -730,6 +730,9 @@
   def get_args(self):
     return self.target.args
 
+  def get_cmd(self):
+    return ""
+
   def get_outputs(self):
     return self.target.outputs