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 c8b29ac6)

Bug: 36056324
Test: builds, boots
Merged-In: Ibdf1ee4e8885680e106df6a9269b6309ddc61af8
Change-Id: Ibdf1ee4e8885680e106df6a9269b6309ddc61af8