Fix NPE when a volume is ejected.

When a removable volume is ejected, StorageManager.getVolumeName() can
return null; deal with this properly in getVolumeName() and all calling
code.

Bug: 170498038
Test: atest CtsScopedStorageHostTest
Change-Id: Ic917836be695e191902706b577b75cc06967e914
3 files changed