Parse and expose end-of-life flag.

Omaha update or noupdate response can include _key=value pairs with
arbitrary data. One of those key can be "_eol" with the one of the
values "supported", "security-only" or "eol" which notifies the device
the end-of-life status of the device with respect to updates. This
information is now exposed via GetEolStatus() to the client so it
can be properly displayed in the UI.

Bug: 27924505
TEST=Added unittest. Run `update_engine_client --eol_status` on link.

Change-Id: Icc15f25b4d0b19cc894f5afc52ac7c43c7818982