More APIs for cache status and behavior.

Add APIs for apps to query their cache usage compared to their
currently allocated quota.  Since an app's private storage may live
on a different storage volume than the primary shared/external
storage, offer APIs to retrieve those values separately.

Add APIs to control two new cache purging behaviors:

-- setCacheBehaviorAtomic() which causes a marked directory and its
contents to be treated as an atomic unit.
-- setCacheBehaviorTombstone() which causes the OS to truncate
files instead of deleting them.

Test: builds, boots
Bug: 33811826, 33965858, 27948817
Change-Id: I45de165623775c359f78b4ee544c2b5831b8d483