releasetools: Use "ota-downgrade=yes" for --override_timestamp.

We used to write a fake post-timestamp into package metadata for
"--override_timestamp", which allows a package to be pushed based on
the fake timestamp. This CL stops doing that by using the designated
"ota-downgrade=yes" instead, but keeping the "--override_timestamp" flag
for backward compatibility.

Now both "--override_timestamp" and "--downgrade" allow pushing packages
with downgrade timestamps. The only differenece is that "--downgrade"
enforces a data wiping, because it's intended a real downgrade (based on
Android version or security patch level). "--override_timestamp" serves
the path of a legit "upgrade", but unfortunately with build timestamps
in reverse order (e.g. the two builds are cut from different branches).

With this CL, we write "post-timestamp" to all packages.

Bug: 77811049
Test: Generate an incremental pakcage with "--override_timestamp".
Test: Generate an incremental pakcage with "--downgrade".
Test: python -m unittest test_ota_from_target_files
Change-Id: I4ae4ce77cb600a06ffd9b15b6d4c730314aa0b79