Initial API shape for storage statistics.

This API is designed to provide both UID-level stats and overall
summary data for a given storage device, as identified by UUID.

The use of UID-level granularity might appear a bit clunky, but it
matches other usage statistics (such as network and battery), and it
allows us to implement it using an extremely fast quota kernel
feature.

A future CL will wire up the implementation to installd.

Test: builds, boots
Bug: 32206268
Change-Id: I7b51877682d0370c2402c19346f57809f0e7ac53