Fix par file zip offsets

The zip file format does support scripts/programs prefixed to the
archive, though many of the offsets are supposed to start from the
beginning of the file. Some tools (python and zipinfo) are able to
read zip files with arbitrary prefixes, but others (libziparchive and
zipdetails) fail.

So pass the file to prefix onto the zip file into merge_zips so that we
can set the offsets from the real start of the file.

Test: m sepolicy_tests  (runs the embedded python interpreter)
Test: zipinfo out/host/linux-x86/bin/sepolicy_tests
Test: zipdetails out/host/linux-x86/bin/sepolicy_tests
Change-Id: If73d4c2465581f7de5aa47959284ecf2059df091