Clean up CMUpdater.

Remove a lot of legacy code (e.g. remainders of incremental ROM support)
and get rid of weird design patterns, such as spawning another process
to get a system property value.

Also make the update check service a little more clever:
- it's now able to be cancelled
- it doesn't fetch changelogs that are later dropped anymore

Change-Id: I07b16a0c3bb55791b0025698ede2c9e0f4e969de