Use base::WriteFully in zip_archive.

We're already linking against libbase but we'll have to add
a libbase dependency to every target that includes libziparchive
as a STATIC_LIBRARY dependency, given that there's no way to
express that what we want (except by adding a LOCAL_WHOLE_STATIC_LIBRARY
dependency on libbase to libziparchive but that seems bad too)

Bug: http://b/21558406
Change-Id: I294ad389a9c61a1134a7bc323da25b0004a8f1e0
(cherry picked from commit e97e66ea)