Pack the update_engine_config.txt when using the A/B OTA updater.

This config file pulled directly from the source specifies the update
payload version supported by the code. When generating an incremental
update payload for the A/B updater, the generated payload format will
match the version supported by the old version.

Bug: 24406755
Test: `make dist` on Brillo.

Change-Id: Ib7ac6828d0d34eb31e786dbaae5d59ecf26c3576