Fix crash issue of deleting expired content when idle

When we delete the expired content on onIdleMaintenance, if the
content is located on unmounted volume, it raises
VolumeNotFoundException. Filter the unmounted volumes.

We don't delete the content on unmounted volumes, change the comments
and update the correct deleted count.

Test: manual
Bug: 176940918
Change-Id: I0b0593972de3d02f67cb013ae329a5005215371f
Merged-In: I0b0593972de3d02f67cb013ae329a5005215371f
(cherry picked from commit 9cb5f55e6b9d59dc873e9a1128a586ed4ca9c968)
1 file changed