Update button could work for non-managed SDK too

[elharo]

Currently we disable the update button when a non-managed SDK is selected, but it would be neat of instead it just worked.

[chanseokoh]

One problem I heard is that, we can't update by gcloud components update for an SDK installed through a package manager such as apt, yum, etc.

[briandealwis]

I wonder if that situation can be detected. Maybe a check that the gcloud directory is writable.