Correct used size of Storage Settings.

Since StorageManager rounded the totalBytes of device storage but
they don't do that rounding for freeBytes.
So in some cases the freeBytes can be greater than totalBytes.

We propose to get totalBytes of device storage directly to calculate
correct privateUsedBytes.

Test: robotest
Bug: 281955532
Change-Id: I4f137c20e7c2f54f4e037d50e81b3176edc1a83a