pdl: Specify that packets must be byte aligned

This also applies to structs, which already say that they inherit the
rules from packets.

Test: n/a.
(cherry picked from https://android-review.googlesource.com/q/commit:98b05329326d4f0b09f8966055e3c194d4d2d774)
Merged-In: Ia692582c3337336f114302caad1ed7728e7d8e4b
Change-Id: Ia692582c3337336f114302caad1ed7728e7d8e4b