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 7ee3a9678e7191c48f0ba4e04792fe97925c1aa1)
diff --git a/tools/releasetools/sign_target_files_apks.py b/tools/releasetools/sign_target_files_apks.py
index 8941e35..f758ae0 100755
--- a/tools/releasetools/sign_target_files_apks.py
+++ b/tools/releasetools/sign_target_files_apks.py
@@ -421,8 +421,8 @@
   temp_file = cStringIO.StringIO()
   certs_zip = zipfile.ZipFile(temp_file, "w")
   for k in mapped_keys:
-    certs_zip.write(k)
-  certs_zip.close()
+    common.ZipWrite(certs_zip, k)
+  common.ZipClose(certs_zip)
   common.ZipWriteStr(output_tf_zip, "SYSTEM/etc/security/otacerts.zip",
                      temp_file.getvalue())