Support Factory OTA for Virtual A/B devices

In Factory OTA, secondary payload should be generated without
virtual_ab flag set, so that update package generator generates
a regular A/B OTA (with snapshot_enabled set to false), and
OTA client writes system_b without snapshots.

Test: apply secondary OTA
Fixes: 138733621
Change-Id: I05d534a12d4a7891ce5597f3c29e55f5185e5a8d