Clear cache space when allocating bytes.

Fleshes out remainder of allocation implementation, where we offer
to clear cached data to satisfy the allocation request.  To prevent
abuse, we never let apps allocate into either the minimum cache space
or low storage space.

Clean up quota APIs to require the caller to pass in the path they're
interested in, and we resolve the underlying filesystem for them.

Defines settings that can be used to tweak the minimum cache space.

Test: builds, boots
Bug: 34690590
Change-Id: I85bc07399f91ee4aa568a8a54c615646bf748ad4