Wire up non-visible volumes, more states.

Adds logic to ExternalStorageProvider to scan non-visible volumes,
such as USB OTG devices.  We use internal paths when surfacing these
volumes, which also optimizes around the FUSE daemon for public
devices.  Also dumps internal state when requested.

VolumeInfo now directly contains DiskInfo, which means it's
snapshotted when sending events, avoiding teardown races.  Switch
notifications to use this DiskInfo directly.

Finish wiring up new volume state, including helper methods to make
it readable/writable state clearer.  Handle disks and volumes with
spaces in their labels.

Bug: 19993667
Change-Id: I5c75e5658a6415976811477aebafee7694bde0f4