releasetools: Use fixed timestamp in otacerts.zip.

We were using zipfile.write() to write system/etc/security/otacerts.zip
when signing for release. It led to unexpected timestamp change in the
generated otacerts.zip and non-idempotent signed images when signing the
same target_files.zip.

Replace with common.ZipWrite() to always use a fixed timestamp.

Bug: 28122968
Change-Id: Ia6cf4b7d380cbf72ed7050ebb60c932dc8826d87
(cherry picked from commit 7ee3a967)