Negatively cache settings and proactively slurp settings into cache.

The settings database cache is tiny (or should be tiny) and can be
slurped into memory.  Once it's in memory and we know we have it all
we can avoid going to disk at all for keys not in the cache.

This is a big percentage of the StrictMode violations & latency.

Change-Id: I649411be0c40d348f58376ccfb3eda059fd69fbc