Make the storage free/total sizes consistent.

We were using a different calculation in the top level view compared to
within the Storage settings. Technically, both are correct (one of them
is aware that we're considering cache as free, one does not). This patch
aligns us on the cache as free strategy.

Change-Id: I9ac26683a4d2a30b77a1da534aa2ddd3d4da6657
Fixes: 37175551
Test: Settings robotest