Use "sys.isolated_storage_snapshot" prop to check for the feature.

StorageManager.hasIsolatedStorage() has already been updated to use
this, so most callers are already using this sys prop. Now, updating
remaining callers to use it as well.

Bug: 122559151
Test: manual
Change-Id: I1e48c00d21d98a47c6625cdf62d0ab69304e03e5