Properly align some packed data structures.

Bug: http://b/31532493

Test: Built with -Waddress-of-packed-member enabled.

These data structures later take the address of their uuid field, which
results in a possibly unaligned access to the underlying data. To
prevent the misalignment, we add a further attribute to ensure that the
base of these packed structures is aligned at least as much as the uuid
data member requires (so we can continue using the address of it for
direct dereferences).

Change-Id: I3ec6945b6ea343bef11326a10343309a863786cf