bpf_shared.h - add asserts for size of structures

This is to make sure we maintain compatibility between C/C++ and Java.

As a side effect we change a uint8 to a uint32, and add 3 explicit padding bytes.

Test: builds
Signed-off-by: Maciej Żenczyowski <maze@google.com>
Change-Id: Icc7f89da8cc8ecb03052cc6489931470a946dc85