releasetools: Specify max_timestamp for secondary payload.

This is a mandatory field in the generated payload.

Bug: 35724498
Test: Generate OTA with --include_secondary. Check the generation
      command for secondary payload.
Change-Id: Ib2e25cc020e294eae5a4be36d6dca297432804b8
Merged-In: Ib2e25cc020e294eae5a4be36d6dca297432804b8
(cherry picked from commit 60481214)