releasetools: Specify SWITCH_SLOT_ON_REBOOT for secondary payload.

The secondary payload should always be applied with
SWITCH_SLOT_ON_REBOOT=0. This CL moves the 'secondary' parameter from
Payload.WriteToZip() to Payload.__init__(). So it can append the flag to
secondary/payload_properties.txt.

Bug: 35724498
Test: Generate an A/B OTA with --include_secondary. Check
      secondary/payload_properties.txt entry in the generated ZIP.
Test: `python -m unittest test_ota_from_target_files`
Change-Id: I816c07ab57a1c8a52eff785801634b8b1cb134d4