Fix broken runtime SetStatsEnabled logic

Previously, Runtime::SetStatsEnabled wouldn't take stats_enabled_
into account when deciding whether or not to increment / decrement
teh stats enabled counter. This resulted in counter underflows and
other errors which caused some CTS tests to fail.

Also added some locking to prevent race conditions.

Bug: 17360878

(cherry picked from commit a98ffd74)

Change-Id: I21d241a58d35bd6a607aa2305c6da81720bd0886