Fix the permission setting in common.ZipWriteStr()

When passing a ZipInfo instance to common.ZipWriteStr(), the
external_attr attribute should not be overwritten unless specified.
We didn't have the issue previously because we were calling
ZipFile.writestr() directly until [1] merged.

[1] commit 2ed665a0.

Bug: http://b/21309935
Change-Id: I374ccd40c174ff9259844f38bdbf187dfe82552d