metricsd: Fix the update channel parsing logic.

libupdate_engine_client returns the channel as "stable-channel",
"dev-channel" or "beta-channel". This CL updates the parsing logic to
match this format.

Bug: 25669566
Test: unit tests
Test: manual: The reported channel matches what update_engine_client
  --show_channel reports.

Change-Id: Ic6e52df892ae0b438f58fc05b178d5844d6674d0