releasetools: Support generating downgrade incremental OTAs.

Add --downgrade flag to ota_from_target_files.py script. It allows
generating an incremental OTA that updates from a newer build to an
older one (based on timestamp comparison). "post-timestamp" line in the
metadata file will be replaced by "ota-downgrade=yes". A data wipe will
always be enforced, so "ota-wipe=yes" will also be included in the
metadata file.

Bug: 26883782
Change-Id: Iaa05f662d948b7ab632a9fbb7051cc3f8bf68c21