releasetools: Support packaging secondary payload.

By default, an A/B OTA package doesn't contain the images for the
secondary slot (e.g. system_other.img). Specifying
"--include_secondary" that's introduced in this CL allows generating
a separate payload that will install secondary slot images. Both
payloads will be added to the generated A/B OTA package.

An example A/B OTA package with secondary payload
  |
  +-- payload.bin
  |
  +-- payload_properties.txt
  |
  +-- secondary/payload.bin
  |
  +-- secondary/payload_properties.txt
  |
  +-- ...

Such a package needs to be applied in a two-stage manner. During the
first stage, the updater applies the primary payload only. Upon
finishing, it reboots the device into the newly updated slot. It then
continues to install the secondary payload to the inactive slot, but
without switching the active slot at the end (needs the matching support
in update_engine, i.e. SWITCH_SLOT_ON_REBOOT flag).

Due to the special install procedure, the secondary payload will be
always generated as a full payload.

Bug: 35724498
Test: Generate full and incremental OTAs with --include_secondary. Check
      the generated OTAs.
Test: python -m unittest test_ota_from_target_files
Change-Id: I975e826bec492e86eb400f99de0c355a32420127