CMUpdater: only list updates for current channel

Steam crossing is bad. CMUpdater should not offer to install M
snapshots over nightlies, and vice versa. Offer only updates that
correspond to the currently installed cm.releasetype. M snapshots only
display newer updates, while nightlies display all available nightlies.

Change-Id: I3b9f445d61815dbec9db9477d460d1b74458bc95