releasetools: Allow skipping postinstall hooks when generating A/B OTAs.

This CL adds a new flag '--skip_postinstall' that allows skipping all
the postinstall hooks when generating an A/B OTA package (default:
False). Note that this discards ALL the hooks, including non-optional
ones. Should only be used if caller knows it's safe to do so (e.g. all
the postinstall work is to dexopt apps and a data wipe will happen
immediately after).

Bug: 73547992
Test: python -m unittest test_ota_from_target_files
Test: Generate a full OTA package for walleye. Examine the generated
      payload.
Change-Id: Ifc069e897b4019605051eabfd221230a6a37867c