releasetools: Make common.ZipWriteStr Python 3 compatible.

Python 2 and 3 behave differently when calling ZipFile.writestr() with
zinfo.external_attr being 0. Python 3 uses `0o600 << 16` as the value
for such a case (since
https://github.com/python/cpython/commit/18ee29d0b870caddc0806916ca2c823254f1a1f9),
which seems to make more sense. Otherwise the entry will end up with
0o000 as the permission bits. This CL updates common.ZipWriteStr to
follow the logic in Python 3, in order to get consistent behavior
between using the two versions.

Bug: 131631303
Test: `python -m unittest test_common.CommonZipTest`
Test: `python3 -m unittest test_common.CommonZipTest`
Change-Id: If8429855d922ef1ad76591f703215a0ce5089f0f