Fix warnings in libziparchive

system/core/include is included in the global include path using
-isystem, which hides all warnings.  zlib.h is included through
system/core/include/ziparchive/zip_archive.h, which was hiding warnings
in it.  Use a #pragma around the call to deflateInit2, it is a macro
that expands to an old-style cast, in preparation for moving from
-isystem to -I.  Also move the ZipString constructor to zip_archive.cc
so it can assert on the length of the string parameter and fix an
implicit conversion from size_t to uint16_t.

Test: m -j native
Bug: 31492149
Change-Id: I74cdad7fe9c723859b5cfbea73c8f27d9d9ca265