Fix incorrect storage size in Storage Settings

From Android S, Storage Settings queries file sizes
from MediaProvider, it's not necessary to count size
data from getExternalStorageStats when calculating
size data.

Bug: 185542752
Test: make RunSettingsRoboTests ROBOTEST_FILTER=testMeasurementCompletedUpdatesPreferences
      manual
      1. Connect device to PC via USB File transfer.
      2. Copy large files from PC to device.
      3. Observe size information in Storage Settings.
Change-Id: I43cbec764e97f5003a4ec981717c9f213a21b614