Invalidate caches when locale changes.

When loading roots for the first time, we're okay using any cached
data from the system, but if the locale changes we need to
force-refresh everything.

Now that we're always using the system cache, we have a nice strong
signal for "empty" versus "not cached" results, so we don't need to
wait around for the first loading pass to finish.

Add logic to invalidate system cache when locale changes, and fix
locking bug.

Bug: 27977906
Change-Id: Ic50083eff360bea887799583f6c9f02c129eec91