Explicit API for allocating disk space.

Now that we're giving apps better guidance around how much cached
disk space they can use, we also need to provide a way to help clear
some of those cached files.  The final logic is coming in a future
CL, but it will be designed to prevent abuse.

Test: newly added CTS tests
Bug: 34690590
Change-Id: I1e46ade0cdabbc33162fc7bfa76abec711992f92