Change storage migration to use quota APIs.

New quota APIs are much faster than trying to measure manually, and
removing this last user of calculateDirectorySize() means we can
remove it once and for all.

(cherry picked from commit fc522c67)

Bug: 36056324
Test: builds, boots
Merged-In: Icdf774cff520a4b7ca6ec210b34a1c5ff85f8110
Change-Id: Icdf774cff520a4b7ca6ec210b34a1c5ff85f8110