Fix 2009-01-01 timestamps in releasetools to always be UTC

The usage of datetime.fromtimestamp previously resulted in the build or
signing machine's local timezone affecting the Unix timestamp ultimately
applied to images generated by add_img_to_target_files. The go/ab build
outputs would use 2009-01-01 00:00 UTC, for example, but local builds
and the release signed images (generated through go/ab-sign) would use
2009-01-01 00:00 PST. This change makes the timestamps always use UTC.

Bug: 80600931
Bug: 80093599
Test: 'm -j droid dist' and verified timestamps in resulting
target_files zip.

Change-Id: Ic2a19591519850c249f78254e1464aa6839bfc6c