Make NOTICE.xml.gz files deterministic

gzip.open produces a .gz file that contains the current time in the
mtime field of the gzip header.  Use the gzip.GzipFile constructor
instead, which takes an mtime parameter, and set it to 0 to get
deterministic output.

Test: m out/target/product/vsoc_x86_64_only/system/etc/NOTICE.xml.gz
Change-Id: I3c5d3b09191b23362e0b36a85f57323ce3c26868