Just round up the data partition size.

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