Volumes know parent disks; unsupported disks.

This is cleaner and more direct than the reverse of having the disk
publish child volume membership.  Rename state constants to match
public API.  Add state representing bad removal.  Make it clear that
volume flags are related to mounting.

Send new unsupported disk event when we finish scanning an entire
disk and have no meaningful volumes.

Splice disk labels into description when known.  Only adoptable
slots are directly visible to apps.

Bug: 19993667
Change-Id: I12fda95be0d82781f70c3d85c039749052dc936b