test_vendor_lib: Add working post delayed task

This upload fixes the post delayed task functionality so that it uses
a custom time stamped event object instead of posting callbacks to the
vendor manager's task runner (which does not support delayed tasks for
MessageLoopForIO).

Bug: 21586676
Change-Id: Ib5d9019d8c8dd5c9ad865df82eac16b0ecf50268