When computing the data offset of an entry in zip file, we used length
of extra field from central directory. That is correct most of the time
but wrong if the extra field in central directory has different length
than the one in local file directory. Since python's zipfile doesn't
provide an API to access local file header, we need to parse local file
header ourselves and extract length of extra field.
An incorrect offset will cause magic mismatch error from update_engine,
as update_engine expects to find uncompressed payload at the recorded
offset.
Test: th, partner verification
Bug: 191443484
Change-Id: Id670cd79b0bd65adffaaa5224ae4f8065d66b358
