GitLab

Devices that use physical media as their primary storage may not
have that storage mounted by the time that MediaProvider is started,
which means we'd snapshot a stale primary storage path.

To fix this, refresh those paths as volumes are mounted or unmounted
over time.  Also fix logic that detects secondary storage devices.

Bug: 22629557
Change-Id: I3c6b057ce082318e1b28e261c953a436da42e36f