Properly align a packed structure.

Bug: http://b/31532493

Taking the address of a packed structure member only guarantees an
alignment of 1, so any use of that address could be misaligned.
Our latest toolchain will generate an error when these packed members
are accessed in an invalid manner. To remedy this, we can mark the
packed structure as having a stricter alignment (so that we can then
further reason about the packed members). We pick uint32_t because that
is the first field for this structure (and the one that is having its
address taken).

Test: Builds successfully with old and updated toolchain.
Change-Id: If98c05097953fbad8437fb04b0e207a85604ae1d