Add a new flag "--disable_fec_computation" for OTA generation

We have added the flag to disable on device fec computation in
brillo_update_payload. Exposed it in ota_from_target_files as well.

Bug: 139723500
Test: generate an incremental package with the new flag
Change-Id: I683e3ec7791963a3c7e141ecb4e5a3fb53fe2108