Remove locks from StateManager to avoid race condition

A race condition is introduced in ValueMetric due to StateManager's
locks. When StateManager processes a log event, it notifies all
listeners (metric producers) if a state has changed. In ValueMetric, a
state change eventually requires querying through a Statemanager
function that is held by the same lock as StateManager::onLogEvent.

To fix this, we can remove the locks in StateManager and use the locks
in StatsLogProcessor to protect StateManager objects. However, we have
to ensure that the path to StateManager function calls all originate
from StatsLogProcessor.

Test: bit statsd_test:*
Bug: 145838132
Change-Id: Ie5b747961ffa4cd5c78e86fa5f78472e29a972dd