Add vdc volume getStorageSize

StorageStatsManager.getTotalBytes currently takes the size of /data and
rounds up to known probable sizes to guess the size of internal storage.
This is not always correct.

Instead, find the device /data is on and get the size of that device.
This should give a more accurate answer.

Bug: 295358118
Test: vdc volume getStorageSize returns storage size
(cherry picked from https://android-review.googlesource.com/q/commit:d73dfd4ab0ff4cbf704034750b2a25832da7fb0c)
Merged-In: I907892041b1ce2cd72092a9877ac34c12bf3f254
Change-Id: I907892041b1ce2cd72092a9877ac34c12bf3f254