Offer to measure disk stats using quotas.

Now we're getting somewhere!  This CL starts measuring disk usage
using quotactl(), which is almost instant and has much lower impact
on flash memory lifetime.

We now grant the per-app cache GID to every launched app, and the
ContextImpl logic that creates cache directories matches the logic
down in installd.

Test: builds, boots, quota stats match manual stats
Bug: 27948817
Change-Id: Ie269a2958ce0e1c17cb74dbfecc791a5c12922cf