[omaha-client] Read Apps and Config from build-info.

/config/build-info/omaha_product_id for app id and
/config/build-info/omaha_build_id for app version.
These two files will be generated by console and they currently do not
exist in any build yet.

Bug: PKG-678 #done
Test: fx shell run fuchsia-pkg://fuchsia.com/omaha_client#meta/omaha_client_service.cmx
Change-Id: Icf2267beb0dcf2719e5f02899053a53c215bba1c