use reflection for readBlob / writeBlob

- This is used only for hidden api, but moving it out from
  CarPackageManager makes it uglier, so keep it there
  but use reflection so that it can be built with public SDK.
- This can bring little bit performance overhead, but
  policy setting is not something that is done often, so
  that should not be a problem.
- For older platforms, alternative of reading / writing byte array
  is also implemented.

Change-Id: Id6b1f642abb0f24ffc137cf8c5cbb2f7aa9f32f5
1 file changed