Define range of GIDs for cached app data.

To support upcoming disk usage calculation optimizations, this change
creates a new GID for each app that will be used to mark its cached
data.  We're allocating these unique GIDs so that we can use
quotactl() to track cached data on a per-app basis.

Test: builds, boots, tests pass
Bug: 27948817
Change-Id: Ic00c39ccedc23d5d43988029e9921679126f8f2d