Add traits to common utils data structures.

Many of our basic data structures are trivially movable using
memcpy() even if they are not trivially constructable, destructable
or copyable.  It's worth taking advantage of this *ahem* trait.

Adding trivial_move_trait to String16 reduces appt running
time on frameworks/base/core/res by 40%!

Change-Id: I630a1a027e2d0ded96856e4ca042ea82906289fe