Offer to measure disk stats using quotas.

Now we're getting somewhere!  This CL starts using quotactl() to read
UID/GID quota statistics when called with FLAG_USE_QUOTA, otherwise
it continues using the old heavy-weight traversal for calculation.

We now set the recently defined per-app GID used for identifying
cached data, and we use the sticky GID bit on cache directories to
ensure that newly created data inherits the GID for tracking
purposes.  For any existing apps during an upgrade, an initial
bootstrapping case will recursively set this new GID.

This change also shuffles around a bunch of the tedious manual
accounting logic so that we exactly match the new quota statistics.

Test: builds, boots, quota stats match manual stats
Bug: 27948817
Change-Id: I8512c3193ce698f197a3f446e625d6a1c74e7649