Instead of trying to be clever by poking at underlying flash part
sizes, rely on the fact that device storage printed on retail
packaging is a power-of-two value.
For a typical device with a 23GiB data partition, this will return
a value of "32GB" which matches the retail packaging.
Test: builds, boots
Bug: 34827187
Change-Id: Ib4cf7f637dffc9238252e1fedcd86dc8b5cf656d