stagefright: extendedstats: Synchronize logging mechanism

Add a lock to all public functions of extendedstats to prevent
clients from modifying the same resource at the same time. For example
two clients can call getLogEntry and without locks one will end
up traversing the vector while the other is modifying it, leading
to a crash.

Change-Id: I908e0ebcc01026c258554214338f43c5d1226eb7