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.

This change also tightens up the implementation of both the cache
and shared GID calculation to ensure that they stay inside the valid
ranges, and includes tests to verify.

Test: builds, boots, tests pass
Bug: 27948817
Change-Id: Ie4377e5aae267f2da39a165888139228995987cb