Adds locks to ConfigManager in statsd.

The ConfigManager can be triggered by Binder threads, so we should
lock any reads or writes to the config managers. We don't need to
lock within StatsService, so those methods simply call the methods
in ConfigManager.

Test: Test that statsd builds and unit tests pass.
Change-Id: I96904c4140a95dc60a419281c8c54f606a443fbc