Generate a trivial dep file for parcelables

It turns out that the SDK tools request this.

Bug: 28091660
Test: unittests pass, added another

Change-Id: Ide4ec53969ad332d5e32dc3d9535076ec2872a5a
diff --git a/aidl.cpp b/aidl.cpp
index d7883b9..d74c376 100644
--- a/aidl.cpp
+++ b/aidl.cpp
@@ -690,7 +690,9 @@
   if (aidl_err == AidlError::FOUND_PARCELABLE && !options.fail_on_parcelable_) {
     // We aborted code generation because this file contains parcelables.
     // However, we were not told to complain if we find parcelables.
-    // Just exit quietly.
+    // Just generate a dep file and exit quietly.  The dep file is for a legacy
+    // use case by the SDK.
+    write_java_dep_file(options, imports, io_delegate, "");
     return 0;
   }
   if (aidl_err != AidlError::OK) {