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