Use "real" free space; refresh on large changes.

For volumes where the OS manages cached data, use the "free space" as
reported by StorageStatsManager, which is the same value shown in
the Settings app and other UI elements.

Also, when the storage space changes significantly, invalidate anyone
who was holding a cached "free space" value.

Test: builds, boots
Bug: 38146029
Change-Id: I4b3a484a8bf32cd137a83f1ea441beca6dc6719a