Tests & stronger guarantees for hidl_* pads.

- Test that pads are zero (for future proofing, also ran tests
  on original implementation and bugfix).
- Use -Wpadded so that compiler guarantees there aren't padding
  bits hiding elsewhere (there aren't)

Bug: 131356202
Test: libhidl_test
  (without fixes, with fixes, and with this CL)

Change-Id: Ib52a16015b0393c104cd984376328cb0da888b03
4 files changed