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 a0136989)

Bug: 36056324
Test: builds, boots
Merged-In: I60bb197ffc161ed24fe4aa8093495c8874095cdb
Change-Id: I60bb197ffc161ed24fe4aa8093495c8874095cdb