use main looper when null Handler is passed to Car

- The current thread's looper may not be ready and
  this leads into test failure for test code as test inst
  thread has Looper but it never loops and this leads into
  not getting any event.
- If Handler is null, always dispatch to main thread by using
  main Looper.

bug: 31632485
Change-Id: I11f90a6df5fc7a0223176a449465baacb381b715
2 files changed