Add spl-downgrade field to ota metadata

In aosp/1581143 , we banned generation of SPL downgrade OTAs. However,
caller of OTA script can still force an SPL downgrade OTA by passing
--spl_downgrade flag. If this flag is specified, we propagate it to OTA
metadata so that GOTA server can properly honor this flag.

Test: th
Change-Id: Ic8cdc850d2210f4149ad9121fa4ed2e5a4f59bcc