StatsManager uses private object for synchronizing

StatsManager used to synchronize on "this", which is dangerous because
an app can get StatsManager and also sychronize on it, which would block
all calls. We now lock on a private final object.

Test: make
Bug: 144315658
Fixes: 144315658
Change-Id: Ia34b1c586b09a48b244cc16dba77fe54e81f62cf